https://leetcode-cn.com/problems/same-tree/submissions/
    递归：

    1.  var isSameTree = function(p, q) {
    2.   if (p === null && q === null) { // 递归退出的条件
    3.     return true
    4.   }
    5.   if (p !== null || q !== null) {
    6.     return false
    7.   }
    8.   if (p.val !== q.val) {
    9.     return false
    10.   }
    11.   return isSameTree(p.left, q.left) && isSameTree(p.right, q.right)
    12. };

    迭代：
    用一个队列维护

    1. var isSameTree = function (p, q) {
    2.   let queue = []
    3.   queue.push(p, q)
    4.   while(queue.length !== null) {
    5.     let tmpp = queue.shift() // 先进先出
    6.     let tmpq = queue.shift()
    7.     if (tmpp === null && tmpq === null) {continue};
    8.     if (tmpp && tmpq && tmpp.val === tmpq.val) {
    9.       queue.push(tmpp.left, tmpq.left) // 同一个位置比较
    10.       queue.push(tmpp.right, tmpq.right)
    11.     }else{
    12.       return false
    13.     }
    14.   }
    15.   return true
    16. }