【LeetCode】Binary Tree Postorder Traversal

/**
 * Definition for binary tree
 * 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) {
       stack<TreeNode*> s;  
       vector<int> result;
    TreeNode *cur;                      //当前结点   
    TreeNode *pre=NULL;       //前一次访问的结点   
    if(root)
    {
    s.push(root);
    }
    while(!s.empty())  
    {  
        cur=s.top();  
        if((cur->left==NULL&&cur->right==NULL)||  
           (pre!=NULL&&(pre==cur->left||pre==cur->right)))  
        {  
        //    cout<<cur->data;  //如果当前结点没有孩子结点或者孩子节点都已被访问过   
              result.push_back(cur->val);
              s.pop();  
            pre=cur;   
        }  
        else  
        {  
            if(cur->right!=NULL)  
                s.push(cur->right);  
            if(cur->left!=NULL)      
                s.push(cur->left);  
        }  
    }  
    return result;
    }
};