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...