leetcode-103.ts

/**
 * 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)
 *     }
 * }
 */

let output = [];

function preOrder(node: TreeNode | null, level = 0) {
  if (node === null) {
    return;
   }
   
  // Root
  if (!Number.isNaN(node.val)) {
    if (Array.isArray(output[level])) {
        output[level].push(node.val);
    } else {
      output[level] = [node.val];
    }
  }
   
  preOrder(node.left, level + 1);

  preOrder(node.right, level + 1);
}
  
function levelOrder(root: TreeNode | null): number[][] {
  // This is ugly, I know
  output = [];
  preOrder(root)
  return output;
};