【一天一大 lee】二叉树的前序遍历 (难度:中等) - Day20201027

输入: [1,null,2,3]
   1
    \
     2
    /
   3

输出: [1,2,3]

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

var preorderTraversal = function(root) {
  if (root === null) return []

  let queue = [root],
    _result = [],
    node = queue.pop()
  while (queue.length || node !== null) {
    while (node != null) {
      _result.push(node.val)
      queue.push(node)
      // 处理左子树
      node = node.left
    }
    node = queue.pop()
    // 处理右子树
    node = node.right
  }
  return _result
}