BST的两个元素被错误交换，在不改变结构的情况下恢复树。
使用中序遍历，分别找到前者 > 后者的第一个和最后一个，进行交换，faster than 100%
/**
* Definition for a binary tree node.
* function TreeNode(val) {
* this.val = val;
* this.left = this.right = null;
* }
*/
/**
* @param {TreeNode} root
* @return {void} Do not return anything, modify root in-place instead.
*/
var recoverTree = function(root) {
var stack = []
var prev = null
var first = null
var second = null
while (root !== null || stack.length !== 0){
if (root !== null){
stack.push(root)
root = root.left
} else{
root = stack.pop()
if(prev !== null && prev.val > root.val){
first = first === null ? prev : first
second = root
}
prev = root
root = root.right
}
}
var temp = first.val
first.val = second.val
second.val = temp
};
