LeetCode Online Judge 题目C# 练习 - Binary Tree Inorder Traversal

        public static List<int> BinaryTreeInorderTraveral(BTNode root)
        {
            List<int> ret = new List<int>();
            BinaryTreeInorderTraveralHelper(root, ret);
            return ret;
        }

        public static void BinaryTreeInorderTraveralHelper(BTNode root, List<int> ret)
        {
            if (root == null)
                return;

            BinaryTreeInorderTraveralHelper(root.Left, ret);
            ret.Add(root.Value);
            BinaryTreeInorderTraveralHelper(root.Right, ret);
        }

代码分析：

　　Inorder Traveral应该没难度吧？但是如果不能用递归就稍稍有点难度了，我相信如果面试问这题，递归肯定不能让面试官满意的。

        public static List<int> BinaryTreeInorderTraverlOpt(BTNode root)
        {
            List<int> ret = new List<int>();
            if (root == null)
                return ret;

            Stack<BTNode> stack = new Stack<BTNode>();
            bool backtrack = false;

            stack.Push(root);

            while (stack.Count > 0)
            {
                BTNode curr = stack.Peek();
                if (curr.Left != null && !backtrack)
                {
                    stack.Push(curr.Left);
                    backtrack = false;
                    continue;
                }

                ret.Add(curr.Value);
                stack.Pop();
                backtrack = true;

                if (curr.Right != null)
                {
                    stack.Push(curr.Right);
                    backtrack = false;
                }
            }

            return ret;
        }

代码分析：

　　Iterative做法，用一个 Stack 帮忙，记得要 backtrack flag 来标记当前是往下走还是往上走。往上走的时候就不能再把 curr.Left 加到 Stack 中了。