see-145. Binary Tree Postorder Traversal

后续遍历：左子树，右子树，根节点

非递归版本：

class Solution {
public:
    vector<int> postorderTraversal(TreeNode* root) {
        vector<int> ans;
        stack<TreeNode*> temp;
        TreeNode* cur = root;
        TreeNode *pre = NULL;
        while(!temp.empty()||cur)
        {
            if(cur)
            {
                temp.push(cur);
                cur = cur->left;
            }
            else
            {
                TreeNode* flag = temp.top();
                if(flag->right&&flag->right!=pre)
                {
                    cur = flag->right;
                }
                else
                {
                    ans.push_back(flag->val);
                    pre=flag;
                    flag->right=NULL;
                    temp.pop();
                }
            }
        }
        return ans;
    }
};

递归版本：

/**
 * 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> table;
        if(root!=NULL)  
            postOrder(table,root);  
        return table; 
    }
    void postOrder(vector<int> &table,TreeNode* root)  {  
        
        if(root->left!=NULL)  
            postOrder(table,root->left); //visit left  
        if(root->right!=NULL)  
            postOrder(table,root->right); //visit right  
      
        table.push_back(root->val);  //visit root  
    }  
};