leetcode-1038.png
这一题的意思，就是累加，只不过是从右边开始累加
中序遍历是 左中右，在这里可以来逆中序来进行遍历 （右中左）
中序遍历递归&非递归写法可以参考 94 题
递归
var bstToGst = function (root) {
let sum = 0
var reverseInOrder = function (root) {
if (root) {
reverseInOrder(root.right)
sum += root.val
root.val = sum
reverseInOrder(root.left)
}
return root
}
return reverseInOrder(root)
};
非递归
var bstToGst = function (root) {
let sum = 0
let stack = []
let current = root
while(current || stack.length){
while(current){
stack.push(current)
current = current.right
}
current = stack.pop()
sum += current.val
current.val = sum
current = current.left
}
return root
};
