Symmetric tree [https://leetcode.com/problems/symmetric-tree]

symmetric_tree.js

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {boolean}
 */
var isSymmetric = function(root) {
    const leftValues = [];
    const rightValues = [];

    getValues(root.left, leftValues, true);
    getValues(root.right, rightValues, false);

    if (leftValues.length !== rightValues.length) {
        return false;
    }

    for (i = 0; i < leftValues.length; i++) {
        if (leftValues[i] !== rightValues[i]) {
            return false;
        }
    }

    return true;
};

var getValues = function(node, values, isLeftDirection) {
    if (!node) {
        values.push(null);
        return;
    }

    values.push(node.val);

    if (isLeftDirection) {
        getValues(node.left, values, isLeftDirection);
        getValues(node.right, values, isLeftDirection);

        return;
    }

    getValues(node.right, values, isLeftDirection);
    getValues(node.left, values, isLeftDirection);
}