二叉树的基本知识
链接:https://programmercarl.com/二叉树理论基础.html
要点:
- 深度优先遍历
- 前序遍历(递归法,迭代法)
- 中序遍历(递归法,迭代法)
- 后序遍历(递归法,迭代法)
- 广度优先遍历
- 层次遍历(迭代法)
由于栈就是递归的一种实现结构,因此前中后序遍历的逻辑可以借助栈使用递归的方式来实现。
由于队列先进先出的特点,广度优先遍历一般使用队列来实现。
递归的思想:
每次写递归,要按照三要素去写:
- 确定递归函数的参数和返回值。
- 确定终止条件。
- 确定单层递归的逻辑。
下面开始具体的完成算法题目。
144. 二叉树的前序遍历
题目链接:https://leetcode.cn/problems/binary-tree-preorder-traversal/
题目难度:简单
文章讲解:https://programmercarl.com/二叉树的递归遍历.html
视频讲解:https://www.bilibili.com/video/BV1Wh411S7xt
题目状态:通过
递归思路:
使用递归的三要素完成递归操作,通过代码就可以看明白。
代码实现:
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode() : val(0), left(nullptr), right(nullptr) {}
* TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
* TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
* };
*/
class Solution {
public:
void traversal(TreeNode *root, vector<int> &res) {
if(root == nullptr) return;
res.push_back(root->val);
traversal(root->left, res);
traversal(root->right, res);
}
vector<int> preorderTraversal(TreeNode* root) {
vector<int> res;
traversal(root, res);
return res;
}
};
栈的思路:
先将根结点放入栈中,然后将右孩子加入栈,再加入左孩子。
为什么要先加入右孩子,在加入左孩子呢?因为这样出栈的时候才是中左右的顺序(可以看上图理解)。
代码实现:
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode() : val(0), left(nullptr), right(nullptr) {}
* TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
* TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
* };
*/
class Solution {
public:
vector<int> preorderTraversal(TreeNode* root) {
stack<TreeNode *> st;
vector<int> res;
if(root == nullptr) return res;
st.push(root);
while(!st.empty()) {
TreeNode *node = st.top();
st.pop();
res.push_back(node->val);
if(node->right) st.push(node->right);
if(node->left) st.push(node->left);
}
return res;
}
};
145. 二叉树的后序遍历
题目链接:https://leetcode.cn/problems/binary-tree-postorder-traversal/
递归思路和上面一样,甚至代码都不太变,直接看代码:
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode() : val(0), left(nullptr), right(nullptr) {}
* TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
* TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
* };
*/
class Solution {
public:
void traversal(TreeNode *root, vector<int> &res) {
if(root == nullptr) return;
traversal(root->left, res);
traversal(root->right, res);
res.push_back(root->val);
}
vector<int> postorderTraversal(TreeNode* root) {
vector<int> res;
traversal(root, res);
return res;
}
};
栈的思想:
将前面前序遍历的进栈顺序“中->右->左”改变一下,变为“中->左->右”,此时出栈的顺序就是“中->右->左”,之后在反转一下,变为“左->右->中”即可。
代码实现:
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode() : val(0), left(nullptr), right(nullptr) {}
* TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
* TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
* };
*/
class Solution {
public:
vector<int> postorderTraversal(TreeNode* root) {
stack<TreeNode *> st;
vector<int> res;
if(root == nullptr) return res;
st.push(root);
while(!st.empty()) {
TreeNode *node = st.top();
st.pop();
res.push_back(node->val);
if(node->left) st.push(node->left);
if(node->right) st.push(node->right);
}
reverse(res.begin(), res.end());
return res;
}
};
94. 二叉树的中序遍历
题目链接:https://leetcode.cn/problems/binary-tree-inorder-traversal/
递归代码实现:
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode() : val(0), left(nullptr), right(nullptr) {}
* TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
* TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
* };
*/
class Solution {
public:
void traversal(TreeNode *root, vector<int> &res) {
if(root == nullptr) return;
traversal(root->left, res);
res.push_back(root->val);
traversal(root->right, res);
}
vector<int> inorderTraversal(TreeNode* root) {
vector<int> res;
traversal(root, res);
return res;
}
};
栈的思路:
采用指针将左孩子全部压入栈,再遍历出来;之后采用指针将右孩子全部压入栈,再遍历出来。
代码实现:
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode() : val(0), left(nullptr), right(nullptr) {}
* TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
* TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
* };
*/
class Solution {
public:
vector<int> inorderTraversal(TreeNode* root) {
vector<int> res;
stack<TreeNode *> st;
TreeNode *cur = root;
while(cur != nullptr || !st.empty()) {
if(cur != nullptr) {
st.push(cur);
cur = cur->left;
} else {
cur = st.top();
st.pop();
res.push_back(cur->val);
cur = cur->right;
}
}
return res;
}
};
102. 二叉树的层序遍历
题目链接:https://leetcode.cn/problems/binary-tree-level-order-traversal/
题目难度:中等
文章讲解:https://programmercarl.com/0102.二叉树的层序遍历.html
视频讲解:https://www.bilibili.com/video/BV1GY4y1u7b2
题目状态:通过
思路:
利用队列实现层序遍历,并通过size
来判断该层是否已经输出完毕,并在输出每一层的某个元素的时候,将该元素的左右孩子压入队列中,直到队列为空,整个循环完毕,层序遍历结束。
代码实现:
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode() : val(0), left(nullptr), right(nullptr) {}
* TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
* TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
* };
*/
class Solution {
public:
vector<vector<int>> levelOrder(TreeNode* root) {
vector<vector<int>> res;
queue<TreeNode *> que;
if(root != nullptr) que.push(root);
while(!que.empty()) {
int size = que.size();
vector<int> vec;
while(size--) {
TreeNode *node = que.front();
que.pop();
vec.push_back(node->val);
if(node->left) que.push(node->left);
if(node->right) que.push(node->right);
}
res.push_back(vec);
}
return res;
}
};
107. 二叉树的层次遍历II
题目链接:https://leetcode.cn/problems/binary-tree-level-order-traversal-ii/
题目难度:中等
文章讲解:https://programmercarl.com/0102.二叉树的层序遍历.html
视频讲解:https://www.bilibili.com/video/BV1GY4y1u7b2
题目状态:通过
思路:
按102.二叉树的层序遍历的思路来,只是在最后反转一下结果即可。
代码实现:
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode() : val(0), left(nullptr), right(nullptr) {}
* TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
* TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
* };
*/
class Solution {
public:
vector<vector<int>> levelOrderBottom(TreeNode* root) {
vector<vector<int>> res;
queue<TreeNode *> que;
if(root != nullptr) que.push(root);
while(!que.empty()) {
int size = que.size();
vector<int> vec;
while(size--) {
TreeNode *node = que.front();
que.pop();
vec.push_back(node->val);
if(node->left) que.push(node->left);
if(node->right) que.push(node->right);
}
res.push_back(vec);
}
reverse(res.begin(), res.end());
return res;
}
};
199. 二叉树的右视图
题目链接:https://leetcode.cn/problems/binary-tree-right-side-view/
题目难度:中等
文章讲解:https://programmercarl.com/0102.二叉树的层序遍历.html
视频讲解:https://www.bilibili.com/video/BV1GY4y1u7b2
题目状态:通过
个人思路:
同样使用层序遍历,不同的是在每层遍历的时候,只需要把每层最后一个元素的值压入数组中,最后返回这个数组。
代码实现:
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode() : val(0), left(nullptr), right(nullptr) {}
* TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
* TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
* };
*/
class Solution {
public:
vector<int> rightSideView(TreeNode* root) {
queue<TreeNode *> que;
vector<int> vec;
if(root != nullptr) que.push(root);
while(!que.empty()) {
int size = que.size();
for(int i = 0; i < size; ++i) {
TreeNode *node = que.front();
que.pop();
if(i == size - 1) vec.push_back(node->val);
if(node->left) que.push(node->left);
if(node->right) que.push(node->right);
}
}
return vec;
}
};
637. 二叉树的层平均值
题目链接:https://leetcode.cn/problems/average-of-levels-in-binary-tree/
题目难度:简单
文章讲解:https://programmercarl.com/0102.二叉树的层序遍历.html
视频讲解:https://www.bilibili.com/video/BV1GY4y1u7b2
题目状态:通过
个人思路:
依旧是层序遍历,在遍历每一层时记录该层的个数以及该层的和,最后求得平均数,返回在一个数组中。
代码实现:
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode() : val(0), left(nullptr), right(nullptr) {}
* TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
* TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
* };
*/
class Solution {
public:
vector<double> averageOfLevels(TreeNode* root) {
vector<double> res;
queue<TreeNode *> que;
if(root != nullptr) que.push(root);
while(!que.empty()) {
int size = que.size();
int len = size;
double sum = 0;
while(size--) {
TreeNode *node = que.front();
que.pop();
sum += node->val;
if(node->left) que.push(node->left);
if(node->right) que.push(node->right);
}
res.push_back(sum / len);
}
return res;
}
};
429. N 叉树的层序遍历
题目链接:https://leetcode.cn/problems/n-ary-tree-level-order-traversal/
题目难度:中等
文章讲解:https://programmercarl.com/0102.二叉树的层序遍历.html
视频讲解:https://www.bilibili.com/video/BV1GY4y1u7b2
题目状态:通过
个人思路:
加个for
循环,将元素的所有孩子都加入队列中。
代码实现:
/*
// Definition for a Node.
class Node {
public:
int val;
vector<Node*> children;
Node() {}
Node(int _val) {
val = _val;
}
Node(int _val, vector<Node*> _children) {
val = _val;
children = _children;
}
};
*/
class Solution {
public:
vector<vector<int>> levelOrder(Node* root) {
vector<vector<int>> res;
queue<Node *> que;
if(root != nullptr) que.push(root);
while(!que.empty()) {
int size = que.size();
vector<int> vec;
while(size--) {
Node *node = que.front();
que.pop();
vec.push_back(node->val);
for(int i = 0; i < node->children.size(); ++i) {
if(node->children[i]) que.push(node->children[i]);
}
}
res.push_back(vec);
}
return res;
}
};
515. 在每个树行中找最大值
题目链接:https://leetcode.cn/problems/find-largest-value-in-each-tree-row/
题目难度:中等
文章讲解:https://programmercarl.com/0102.二叉树的层序遍历.html
视频讲解:https://www.bilibili.com/video/BV1GY4y1u7b2
题目状态:通过
个人思路:
加个max
,若该层有比max
大的元素,该元素的值就是max
,最后在数组中存入该层的max
即可。
代码实现:
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode() : val(0), left(nullptr), right(nullptr) {}
* TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
* TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
* };
*/
class Solution {
public:
vector<int> largestValues(TreeNode* root) {
queue<TreeNode *> que;
vector<int> res;
if(root != nullptr) que.push(root);
while(!que.empty()) {
int size = que.size();
int max = INT_MIN;
while(size--) {
TreeNode *node = que.front();
que.pop();
max = max > node->val ? max : node->val;
if(node->left) que.push(node->left);
if(node->right) que.push(node->right);
}
res.push_back(max);
}
return res;
}
};
116. 填充每个节点的下一个右侧节点指针
题目链接:https://leetcode.cn/problems/populating-next-right-pointers-in-each-node/
题目难度:中等
文章讲解:https://programmercarl.com/0102.二叉树的层序遍历.html
视频讲解:https://www.bilibili.com/video/BV1GY4y1u7b2
题目状态:通过
个人思路:
定义一个leftmost
来存储每层最左边的元素,遍历每层元素时,将leftmost
的next
指向其后面一个元素,并更新leftmost
为当前元素,直到该层遍历完。
代码实现:
/*
// Definition for a Node.
class Node {
public:
int val;
Node* left;
Node* right;
Node* next;
Node() : val(0), left(NULL), right(NULL), next(NULL) {}
Node(int _val) : val(_val), left(NULL), right(NULL), next(NULL) {}
Node(int _val, Node* _left, Node* _right, Node* _next)
: val(_val), left(_left), right(_right), next(_next) {}
};
*/
class Solution {
public:
Node* connect(Node* root) {
queue<Node *> que;
Node *leftmost;
if(root != nullptr) {
que.push(root);
leftmost = root;
}
while(!que.empty()) {
int size = que.size();
for(int i = 0; i < size; ++i) {
Node *node = que.front();
que.pop();
if(i > 0) {
leftmost->next = node;
}
leftmost = node;
if(node->left) que.push(node->left);
if(node->right) que.push(node->right);
}
}
return root;
}
};
117. 填充每个节点的下一个右侧节点指针II
题目链接:https://leetcode.cn/problems/populating-next-right-pointers-in-each-node-ii/
题目难度:中等
文章讲解:https://programmercarl.com/0102.二叉树的层序遍历.html
视频讲解:https://www.bilibili.com/video/BV1GY4y1u7b2
题目状态:通过
和上一次思路一样,甚至实现代码都一样。
代码实现:
/*
// Definition for a Node.
class Node {
public:
int val;
Node* left;
Node* right;
Node* next;
Node() : val(0), left(NULL), right(NULL), next(NULL) {}
Node(int _val) : val(_val), left(NULL), right(NULL), next(NULL) {}
Node(int _val, Node* _left, Node* _right, Node* _next)
: val(_val), left(_left), right(_right), next(_next) {}
};
*/
class Solution {
public:
Node* connect(Node* root) {
queue<Node *> que;
Node *leftmost;
if(root != nullptr) {
que.push(root);
leftmost = root;
}
while(!que.empty()) {
int size = que.size();
for(int i = 0; i < size; ++i) {
Node *node = que.front();
que.pop();
if(i > 0) {
leftmost->next = node;
}
leftmost = node;
if(node->left) que.push(node->left);
if(node->right) que.push(node->right);
}
}
return root;
}
};
104. 二叉树的最大深度
题目链接:https://leetcode.cn/problems/maximum-depth-of-binary-tree/
题目难度:简单
文章讲解:https://programmercarl.com/0102.二叉树的层序遍历.html
视频讲解:https://www.bilibili.com/video/BV1GY4y1u7b2
题目状态:通过
个人思路:
层序遍历,记录有几层,层数就是二叉树的最大深度。
代码实现:
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode() : val(0), left(nullptr), right(nullptr) {}
* TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
* TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
* };
*/
class Solution {
public:
int maxDepth(TreeNode* root) {
int high = 0;
queue<TreeNode *> que;
if(root != nullptr) que.push(root);
while(!que.empty()) {
int size = que.size();
high++;
while(size--) {
TreeNode *node = que.front();
que.pop();
if(node->left) que.push(node->left);
if(node->right) que.push(node->right);
}
}
return high;
}
};
111. 二叉树的最小深度
题目链接:https://leetcode.cn/problems/minimum-depth-of-binary-tree/
题目难度:简单
文章讲解:https://programmercarl.com/0102.二叉树的层序遍历.html
视频讲解:https://www.bilibili.com/video/BV1GY4y1u7b2
题目状态:通过
思路:
当一个节点既没有左孩子也没有右孩子时,这个节点就是叶子节点,而根结点到叶子节点中间的节点数就是两者之间的高度。
代码实现:
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode() : val(0), left(nullptr), right(nullptr) {}
* TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
* TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
* };
*/
class Solution {
public:
int minDepth(TreeNode* root) {
int high = 0;
queue<TreeNode *> que;
if(root != nullptr) que.push(root);
while(!que.empty()) {
int size = que.size();
high++;
while(size--) {
TreeNode *node = que.front();
que.pop();
if(node->left) que.push(node->left);
if(node->right) que.push(node->right);
if(!node->left && !node->right) return high;
}
}
return high;
}
};
标签:第十一天,node,right,TreeNode,val,随想录,que,二叉树,left
From: https://www.cnblogs.com/frontpen/p/18303686