Binary Tree Level Order Traversal问题及解法

问题描述：

Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).

示例：
Given binary tree [3,9,20,null,null,15,7],

    3   / \  9  20    /  \   15   7

return its level order traversal as:

[  [3],  [9,20],  [15,7]]

问题分析：

层次遍历，顾名思义就是树的广度优先遍历。

过程详见代码：

/** * 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 == NULL) return res;bl(root, res, 0);return res;}void bl(TreeNode* root, vector<vector<int>>& res, int depth){if (root == NULL) return;if (res.size() <= depth)res.push_back(vector<int>());res[depth].push_back(root->val);bl(root->left, res, depth + 1);bl(root->right, res, depth + 1);}};