前端-二叉树的前序遍历

/*
 * @lc app=leetcode.cn id=144 lang=typescript
 *
 * [144] 二叉树的前序遍历
 */

// @lc code=start
/**
 * Definition for a binary tree node.
 * class TreeNode {
 *     val: number
 *     left: TreeNode | null
 *     right: TreeNode | null
 *     constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
 *         this.val = (val===undefined ? 0 : val)
 *         this.left = (left===undefined ? null : left)
 *         this.right = (right===undefined ? null : right)
 *     }
 * }
 */

function preorderTraversal(root: TreeNode | null, res: Array<any> = []): number[] {
  if (!root) {
    return res;
  }
  res.push(root.val);
  preorderTraversal(root.left, res);
  preorderTraversal(root.right, res);
  return res;
};
// @lc code=end