LeetCode 145. Binary Tree Postorder Traversal

先自己在纸上画棵树，发现可以用stack实现迭代版的后序遍历，最开始先把root压入栈。另外维护一个栈，记录对应节点出栈的次数，如果是1，表明左右节点已输出，就输出；如果为0，说明左右节点还未压入栈，就把次数加1，且把左右节点压入栈。

/**
* 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<int> postorderTraversal(TreeNode* root) {
vector<int> res;
if(!root) return res;
stack<TreeNode*> stk;
stack<int> time;
stk.push(root);
time.push(0);
while(!stk.empty()){
TreeNode* top = stk.top();
int tmp = time.top();
if(tmp == 0){
++tmp; time.pop(); time.push(tmp);
if(top->right) {stk.push(top->right);time.push(0);}
if(top->left) {stk.push(top->left);time.push(0);}
}
else{
res.push_back(top->val);
stk.pop();time.pop();
}

}
return res;
}
};