[LeetCode Hot 100] LeetCode102. 二叉树的层序遍历

题目描述

思路

方法一：递归

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();
        level(root, 0, res);
        return res;
    }
    private void level(TreeNode node, int curLevel, List<List<Integer>> res) {
        if (node == null) return;
        if (res.size() == curLevel) {
            List<Integer> list = new ArrayList<>();
            list.add(node.val);
            res.add(list);
        } else {
            res.get(curLevel).add(node.val);
        }
        level(node.left, curLevel + 1, res);
        level(node.right, curLevel + 1, res);
    }
}
class Node{
    TreeNode node;
    int level;
    Node (TreeNode node, int level) {
        this.node = node;
        this.level = level;
    }
}

方法二：迭代

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();
        Deque<TreeNode> queue = new ArrayDeque<>();

        if (root == null) return res;
        queue.offer(root);
        while (!queue.isEmpty()) {
            List<Integer> level = new ArrayList<>();
            int size = queue.size();
            for (int i = 0; i < size; i ++) {
                TreeNode node = queue.poll();
                level.add(node.val);
                if (node.left != null) queue.offer(node.left);
                if (node.right != null) queue.offer(node.right);
            }
            res.add(level);
        }
        return res;
     }
}