二叉树后序遍历

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/**
 * 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> result;
        if(root == NULL)
            return result;
        stack<TreeNode*> s1;
        stack<TreeNode*> s2;
        TreeNode* p = root;
        s1.push(p);
        while(!s1.empty())
        {
            p = s1.top();
            s2.push(p);
            s1.pop();
            if(p->left)
                s1.push(p->left);
            if(p->right)
                s1.push(p->right);
        }
        while(!s2.empty())
        {
            result.push_back(s2.top()->val);
            s2.pop();
        }
        return result;
    }
};

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

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