Binary Tree Level Order Traversal

【题目描述】

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

For example:
Given binary tree {3,9,20,#,#,15,7},

    3   / \  9  20    /  \   15   7

return its level order traversal as:

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

【思路】

参考了disscuss里的思路，使用了队列来记录遍历的节点，然后再用一个vector来记录每一层的节点的值，记得要用push_back()函数在vector的末端插入值。

【我的代码】

/** * 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) {        if(root==NULL) return {};        vector<vector<int>> ans;        queue<TreeNode*> q;        q.push(root);        while(!q.empty()){            vector<int> p;            int len=q.size();            for(int i=0;i<len;i++){                TreeNode* node=q.front();                p.push_back(node->val);                if(node->left!=NULL) q.push(node->left);                if(node->right!=NULL) q.push(node->right);                q.pop();            }            ans.push_back(p);        }        return ans;    }};