145. Binary Tree Postorder Traversal

/** * 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> ret;        stack<TreeNode*> s;        TreeNode *p=root;        TreeNode *pre=NULL;        while(p||!s.empty())        {            if(p)            {                s.push(p);                p=p->left;            }            else            {                p=s.top();                if(p->right&&p->right!=pre)                {                    p=p->right;                    s.push(p);                    p=p->left;                }                else                {                    s.pop();                    ret.push_back(p->val);                    pre=p;                    p=NULL;                }            }        }        return ret;    }};