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

Complexity:

• Time complexity: O(n)
• Space complexity: O(n)

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
function inorderTraversal(root) {
    let res = [];
    helper(root, res);
    return res;
}

function helper(root, res) {
    if (!root) return;
    if (root.left) helper(root.left, res);
    res.push(root.val);
    if (root.right) helper(root.right, res);
}

Solution

Complexity:

• Time complexity: O(n)
• Space complexity: O(n)

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[]}
 */
function inorderTraversal(root) {
    let stack = [];
    let now = root;
    let res = [];

    while (now || stack.length) {
        while (now) {
            stack.push(now);
            now = now.left;
        }
        now = stack.pop();
        res.push(now.val);
        now = now.right;
    }

    return res;
}