LintCode - Binary Tree Postorder Traversal

解法一 recursion

class Solution {
public:
    vector<int> res;
    vector<int> postorderTraversal(TreeNode * root) {
        res.clear();
        helper(root);
        return res;
    }
    void helper(TreeNode* root){
        if(!root) return;
        helper(root->left);
        helper(root->right);
        res.push_back(root->val);
    }
};

解法二 non-recursion

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