leetcode第144题二叉树的前序遍历

题目： 给你二叉树的根节点 root ，返回它节点值的 前序 遍历。 题目链接

• 递归法

/**
 * 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) {
  let result = [];
  if (root)
    travse(root, result);
  return result;
};

var travse = (root, result) => {
  result.push(root.val);
  if (root.left != null)
    travse(root.left, result);
  if (root.right != null)
    travse(root.right, result);
}

• 迭代法

var preorderTraversal = function(root) {
   let stack = [];
   let result = [];
   if (root)
    stack.push(root);
   while (stack.length) {
    let curr = stack.shift();
    result.push(curr.val);
    if(curr.right != null)
      stack.unshift(curr.right);
    if (curr.left != null)
      stack.unshift(curr.left);
   }
   return result;
};