题目

从上到下按层打印二叉树，同一层的节点按从左到右的顺序打印，每一层打印到一行。

限制

节点总数 <= 1000

思路

使用队列来做，dfs，每次看一行

代码

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> res;
        if (!root) return res;

        queue<TreeNode *> q;
        q.push(root);
        while (q.size()) {
            int len = q.size();
            vector<int> level;
            while (len -- ) {
                auto t = q.front();
                q.pop();
                level.push_back(t -> val);
                if (t -> left) q.push(t -> left);
                if (t -> right) q.push(t -> right);
            }
            res.push_back(level);
        }
        return res;
    }
};