Binary Tree Level Order Traversal（二叉树层序遍历）

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]]

题目很基础啊，就是给一个二叉树按照层依次输出，思路：满二叉树的子节点个数是父节点的2倍

cpp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) {                std::vector<TreeNode*> fatherNodes; //存储父节点        std::vector<TreeNode*> childNodes;  //存储子节点        std::vector<vector<int>> result; //返回容器        if(!root) return result;  //空值返回        //插入根节点的值        std::vector<int> temp;        temp.push_back(root->val);        result.push_back(temp);                fatherNodes.push_back(root);  //插入根节点        //循环父节点数组        for (int i = 0; i < fatherNodes.size(); i++) {            //将左右子节点插入子节点数组（NULL值也插入）            childNodes.push_back(fatherNodes[i]->left);            childNodes.push_back(fatherNodes[i]->right);            //判断是否满足满二叉树性质（子节点个数是上一次父节点个数的2倍）            if(childNodes.size() == 2 * fatherNodes.size())            {                //清空父节点数组                fatherNodes.clear();                                //准备插入当前层非空子节点                std::vector<int> levelNodeValues;                for (int j = 0; j < childNodes.size(); j++) {                                        if(childNodes[j] != NULL)                    {                        levelNodeValues.push_back(childNodes[j]->val);                        fatherNodes.push_back(childNodes[j]);                    }                                    }                                //清理子节点数组                childNodes.clear();                                //插入容器中                if (levelNodeValues.size())                    result.push_back(levelNodeValues);                //清理层节点值数组                levelNodeValues.clear();                                i = -1; //重新从0开始循环                        }                    }                return result;    }};