刷题No10 Leetcode Binary Tree Leve

Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).
ExampleGiven binary tree {3,9,20,#,#,15,7},
  3
  /
  9 20
  / \
15 7
return its level order traversal as:[ [3], [9,20], [15,7]]

vector<vector<int>> levelOrder(TreeNode *root) {
    // write your code here
    queue<TreeNode *> Queue;
    vector<vector<int>> levOrder;
    TreeNode *node;
    if (root == NULL)
        return levOrder;
    Queue.push(root);
    while (!Queue.empty()) {
        vector<int> level;
        int size = Queue.size();
        for (int i = 0; i<Queue.size(); i++) {
            node = Queue.front();
            Queue.pop();
            level.push_back(node->val);
            if (node->left != NULL)
                Queue.push(node->left);
            if (node->right != NULL)
                Queue.push(node->right);
        }
        levOrder.push_back(level);

    }
    return levOrder;
}

int size = Queue.size();
此句话用的十分巧妙，通过在未添加新节点时就记录好此时的队列的长度。便可在下面的循环中恰好循环到一层结束的时候。
同时由于下面会对队列进行操作，若不实现将其存入size中，将会导致Queue.size()发生变化。
启示：有效的存储某些临时变量，可将复杂的操作简单化。
Leetcode Binary Tree Level Order Traversal 2
return its level order traversal as:[[15,7],[9,20],[3]]

 vector<vector<int>> levelOrderBottom(TreeNode *root) {
        queue<TreeNode *> Queue;
        vector<vector<int>> levOrder;
        TreeNode *node;
        if(root==NULL)
            return levOrder;
        Queue.push(root);
        while(!Queue.empty()){
            vector<int> level;
            int size=Queue.size();
            for(int i=0;i<size;i++){
                node=Queue.front();
                Queue.pop();
                level.push_back(node->val);
                if(node->left!=NULL)
                    Queue.push(node->left);
                if(node->right!=NULL)
                    Queue.push(node->right);
            }
            levOrder.push_back(level);

        }
        reverse(levOrder.begin(),levOrder.end());
        return levOrder;
    }

reverse(levOrder.begin(),levOrder.end());将数组前后反转
sort(levOrder.begin(),levOrder.end());将数组排序
需添加头文件：
#include <algorithm>