Binary Tree Level Order Traversal II

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概述

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

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

return its bottom-up level order traversal as:

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

/**
* 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>> levelOrderBottom(TreeNode* root)
{
vector<vector<int>> ret;
if(NULL == root)
return ret;
queue<TreeNode *> q;
q.push(root);
while(!q.empty())
{
vector<int> level;
for(int i = 0, n = q.size(); i < n; ++ i)
{
TreeNode *temp = q.front();
q.pop();
if(temp -> left != NULL)
q.push(temp -> left);
if(temp -> right != NULL)
q.push(temp -> right);
level.push_back(temp -> val);
}
ret.push_back(level);
}
reverse(ret.begin(), ret.end());
return ret;
}
};