LeetCode - 94. 二叉树的中序遍历

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

    const traversal = node => {
        if (node) {
            
            if (node.left) {
                traversal(node.left);
            }
            
            ret.push(node.val);
            
            if (node.right) {
                traversal(node.right);
            }
        }
    }
    traversal(root);
    return ret;
    
};