[LeetCode] Binary Tree Level Order Traversal II

iven 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,#,#,15,7},

    3
   / 
  9  20
    /  
   15   7

return its bottom-up level order traversal as:

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

思路：思路和Binary Tree Level Order Traversal相同，在最后的时候reverse.

/**
 * 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> > result;
        if (root == NULL) return result;
        
        vector<int> level;
        
        queue<TreeNode* > q;
        q.push(root);
        q.push(0);
        
        while(!q.empty()) {
            TreeNode* p = q.front();
            q.pop();
            if (p != NULL) {
                level.push_back(p->val);
                if (p->left) q.push(p->left);
                if (p->right) q.push(p->right);
            } else {
                result.push_back(level);
                level.clear();
                
                if (!q.empty())
                    q.push(0);
            }
        }
        
        reverse(result.begin(), result.end());
        
        return result;
    }
};