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;
    }
};