leetCode(22):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 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) {
        queue<TreeNode*> nodes;
    	vector< vector<int> > result;
    	vector<int> tmp;
    	if(NULL==root)
    		return result;
    	nodes.push(root);
    	while(!nodes.empty())
    	{
    		int length=nodes.size();
    		int i=0;
    		while(i<length)
    		{//弹出本层所有结点后，跳出循环，length的值在循环时是不会改变的
    			TreeNode* tmpNode=nodes.front();
    			tmp.push_back(tmpNode->val);
    			if(tmpNode->left)
    				nodes.push(tmpNode->left);
    			if(tmpNode->right)
    				nodes.push(tmpNode->right);
    			nodes.pop();
    			i++;
    		}
    		result.push_back(tmp);
    		tmp.clear();
    	}
    	reverse(result.begin(),result.end());
    	return result;
    }
};

又是层序遍历的方式，层序遍历反复出现过多次：求二叉树的最小深度、右侧能看到的结点、统计特殊形式的二叉树结点个数...