leetcode 145 二叉树的后序遍历

//解法一：递归版算法
/**
 * 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;
        
        postorder(root,ret);

        return ret;
    }

    void postorder(TreeNode* root,vector<int>& v)
    {
         if(root!=NULL)
         {
             postorder(root->left,v);
             postorder(root->right,v);
             v.push_back(root->val);
         }
    }
};

 

//非递归算法：
//Time:O(n) Space:O(n)
/**
 * 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* pre=NULL;

        while(root!=NULL||!s.empty())
        {
            if(root!=NULL)
            {
                s.push(root);
                root=root->left;
            }
            else
            {
                root=s.top();
                root=root->right;

                if(root==NULL||root==pre)
                {
                    pre=s.top();
                    s.pop();
                    ret.push_back(pre->val);
                    root=NULL;
                }
            }
        }

        return ret;
    }
};