二叉树的前序遍历

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题目类型: Tree

题目

手写二叉树的先序遍历.

示例

输入: TreeNode 实例

输出: 每个 TreeNode 节点的 val 组成的数组

题解

这没啥可说的了, 基本功.

迭代版

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

  const stack = [root]
  const res = []

  while (stack.length !== 0) {
    const curr = stack.pop()

    res.push(curr.val)
    if (curr.right !== null) stack.push(curr.right)
    if (curr.left !== null) stack.push(curr.left)
  }

  return res
}

递归版

var preorderTraversal = function (root) {
  const arr = []

  const bst = function (root) {
    if (root) {
      arr.push(root.val)
      bst(root.left)
      bst(root.right)
    }
  }

  bst(root)
  return arr
}