[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]

]



这道题是求二叉树从下往上的层次遍历，与Binary Tree Level Order Traversal十分相似，只是在最后一步将ans反转一下就可以了。

直接贴上代码了：

/**
 * 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) {
        vector<vector<int> > ans;
        queue<TreeNode*> q;
        if (root == NULL)
            return ans;
        q.push(root);
        int nextCount = 0;
        int curCount = 1;
        vector<int> temp;
        while (!q.empty()){
            TreeNode* p = q.front();
            q.pop();
            temp.push_back(p->val);
            if (p->left){
                q.push(p->left);
                nextCount++;
            }
            if (p->right){
                q.push(p->right);
                nextCount++;
            }
            curCount--;
            if (!curCount){
                ans.push_back(temp);
                temp=vector<int>();
                curCount = nextCount;
                nextCount = 0;
            }
        }
        reverse(ans.begin(), ans.end());
        return ans;
    }
};