[LeetCode] DFS - 103. Binary Tree Zigzag Level Order Traversal

문제

Given the root of a binary tree, return the zigzag level order traversal of its nodes' values. (i.e., from left to right, then right to left for the next level and alternate between).

 

Example 1:

Input: root = [3,9,20,null,null,15,7]
Output: [[3],[20,9],[15,7]]

Example 2:

Input: root = [1]
Output: [[1]]

Example 3:

Input: root = []
Output: []

 

Constraints:

• The number of nodes in the tree is in the range [0, 2000].
• -100 <= Node.val <= 100

코드

/**
 * 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 {number[][]}
 */
const zigzagLevelOrder = (root) => {
  // traverse: 3, 20, 9, 15, 17(zigzag)
  let res = [];

  const go = (node, lvl) => {
    if (node == null) return;
    if (res[lvl] == null) res[lvl] = [];

    if (lvl % 2 === 0) {
      res[lvl].push(node.val);
    } else {
      res[lvl].unshift(node.val);
    }

    go(node.left, lvl + 1);
    go(node.right, lvl + 1);
  };

  go(root, 0);
  return res;
};