LeetCode: Binary Tree Postorder Traversal

跟inorder差不多

/**
 * Definition for binary tree
 * 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> res;
        stack<TreeNode *> S;
        if (!root) return res;
        S.push(root);
        while (!S.empty()) {
            TreeNode *top = S.top();
            if (!top->left && !top->right) {
                res.push_back(top->val);
                S.pop();
            }
            else {
                if (top->right) {
                    S.push(top->right);
                    top->right = NULL;
                }
                if (top->left) {
                    S.push(top->left);
                    top->left = NULL;
                }
            }
        }
        return res;
    }
};

 C#

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left;
 *     public TreeNode right;
 *     public TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<int> PostorderTraversal(TreeNode root) {
        List<int> ans = new List<int>();
        Stack<TreeNode> S = new Stack<TreeNode>();
        if (root == null) return ans;
        S.Push(root);
        while (S.Count > 0) {
            TreeNode peek = S.Peek();
            if (peek.left == null && peek.right == null) {
                ans.Add(peek.val);
                S.Pop();
            }
            else {
                if (peek.right != null) {
                    S.Push(peek.right);
                    peek.right = null;
                }
                if (peek.left != null) {
                    S.Push(peek.left);
                    peek.left = null;
                }
            }
        }
        return ans;
    }
}