94. Binary Tree Inorder Traversal

94. Binary Tree Inorder Traversal

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

Example

Input: [1,null,2,3]
   1
    \
     2
    /
   3

Output: [1,3,2]

Follow up

Recursive solution is trivial, could you do it iteratively?

Solution

• ☝️

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        Stack<TreeNode> stack = new Stack<>();
        List<Integer> res = new ArrayList<>();
        while (root != null || !stack.empty()) {
            while(root != null) {
                stack.push(root);
                root = root.left;
            }
            TreeNode node = stack.pop();
            res.add(node.val);
            root = node.right;
        }
        return res;
    }
}