[leetcode]Binary Tree Level Order Traversal II

 

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,#,#,15,7},

    3
   / \
  9  20
    /  \
   15   7

 

return its bottom-up level order traversal as:

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

/**
 * Definition for binary tree
 * 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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        stack<vector<int>> s;
        if(!root) return vector<vector<int>>();
        
        queue<TreeNode*> q1,q2;
        q1.push(root);
        
        TreeNode *cur;
        vector<int> tmp;
        
        while(!q1.empty()){
            tmp.clear();
            while(!q1.empty()){
                cur = q1.front();
                q1.pop();
                
                tmp.push_back(cur -> val);
                if(cur -> left) q2.push(cur -> left);
                if(cur -> right) q2.push(cur -> right);
            }
            s.push(tmp);
            swap(q1, q2);
        }
        
        vector<int> curVec;
        vector<vector<int>> result;
        while(!s.empty()){
            curVec = s.top();
            s.pop();
            result.push_back(curVec);
        }
        return result;
        
    }
        
    
};