94 Binary Tree Inorder Traversal

94. Binary Tree Inorder Traversal

1. Question

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

For example: Given binary tree[1,null,2,3],

   1
    \
     2
    /
   3

return[1,3,2].

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

2. Implementation

(1) Morris Tree Traversal

class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        if (root == null) {
            return res;
        }

        TreeNode curNode = root, preNode = null;

        while (curNode != null) {
            if (curNode.left == null) {
                res.add(curNode.val);
                curNode = curNode.right;
            }
            else {
                preNode = curNode.left;
                while (preNode.right != null && preNode.right != curNode) {
                    preNode = preNode.right;
                }

                if (preNode.right == null) {
                    preNode.right = curNode;
                    curNode = curNode.left;
                }
                else {
                    res.add(curNode.val);
                    preNode.right = null;
                    curNode = curNode.right;
                }
            }
        }
        return res;
    }
}

(2) Iteration

public class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
         List<Integer> res = new ArrayList<>();

        // Method 1: Iterative Stack
        TreeNode curNode = root;
        Stack<TreeNode> stack = new Stack<>();

        while (curNode != null || !stack.isEmpty()) {
            if (curNode != null) {
                stack.push(curNode);
                curNode = curNode.left;
            }
            else {
                curNode = stack.pop();
                res.add(curNode.val);
                curNode = curNode.right;
            }
        }
        return res;
    }
}

3. Time & Space Complexity

(1) Morris Tree Traversal: 时间复杂度: O(n), 空间复杂度: O(1)

(2) Iteration: 时间复杂度: O(n), 空间复杂度：O(h)