Leetcode: Binary Tree Postorder Traversal

Given a binary tree, return the postorder traversal of its nodes' values.

For example:
Given binary tree {1,#,2,3},

   1
    \
     2
    /
   3

 

return [3,2,1].

Note: Recursive solution is trivial, could you do it iteratively?

分析：用两个栈，一个栈A用于扩展子树，另一个B栈用于保存结果。入栈A的顺序是先左子树后右子树，从栈A出的元素被压入栈B。代码如下：

class Solution {
public:
    vector<int> postorderTraversal(TreeNode *root) {
        vector<int> res;
        if(!root) return res;
        stack<TreeNode *> s;
        stack<TreeNode *> output;
        
        TreeNode * cur;
        
        s.push(root);
        while(!s.empty()){
            cur = s.top();
            output.push(cur);
            s.pop();
            if(cur->left) s.push(cur->left);
            if(cur->right) s.push(cur->right);
        }
        while(!output.empty()){
            cur = output.top();
            res.push_back(cur->val);
            output.pop();
        }
        return res;
    }
};