leetcode111-二叉树的最小深度

111. 二叉树的最小深度

这道题相比 104. 二叉树的最大深度 还是难上一些的，但也不算太难。

BFS

/**
 * 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) {
        if(root==nullptr) return 0;
        queue<TreeNode*> qqq;;
        qqq.push(root);
        int depth=0;
        while(!qqq.empty())
        {
            depth++;
            int size=qqq.size();
            for(int i=0;i<size;i++)
            {
                TreeNode *t=qqq.front();
                qqq.pop();
                if(!t->left&&!t->right)  return depth;
                if(t->left) qqq.push(t->left);
                if(t->right) qqq.push(t->right);
            }
        }
        return depth;
    }
};

DFS

/**
 * 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) {
        if(root==nullptr) return 0;
        if(root->left==nullptr&&root->right==nullptr) return 1;
        if(!root->left||!root->right) return minDepth(root->left)+minDepth(root->right)+1;
        /*
        if(root->left&&root->right==nullptr) return minDepth(root->left)+1;
        if(root->right&&root->left==nullptr) return minDepth(root->right)+1;
        */
        return min(minDepth(root->left),minDepth(root->right))+1;
    }
};