题解

BFS

  1. var levelOrder = function(root) {
  2.     return resolve2(root)
  3. };
  4. function resolve2(node) {
  5.     if (!node) return []
  6.     let queue = [node]
  7.     let result = []
  8.     while (queue.length) {
  9.         const lvlen = queue.length
  10.         result.push([])
  11.         for (let i = 0; i < lvlen; i++) {
  12.             node = queue.shift()
  13.             result[result.length - 1].push(node.val)
  14.             if (node.left) queue.push(node.left)
  15.             if (node.right) queue.push(node.right)
  16.         }
  17.     }
  19.     return result
  20. }

递归

var levelOrder = function(root) {
    return resolve2(root)
};

function resolve(node, level = 0, result = []) {
    if (!node) return result
    result[level] ??= []
    result[level].push(node.val)
    resolve(node.left, level + 1, result)
    resolve(node.right, level + 1, result)
    return result
}