116-填充每个节点的下一个右侧节点指针

var connect = function(root) {
    if (!root) return null    var stack = [root]
    while(stack.length) {
        var _stack = [...stack,null]
        stack = []
        var pre = _stack.shift()
        while(_stack.length || pre) {
            pre.left && stack.push(pre.left)
            pre.right && stack.push(pre.right)
            pre.next = _stack.shift()
            pre = pre.next        }
    }
    return root};

while(pre) {
        var cur = pre
        while(cur) {
             cur.left.next = cur.right             cur.right.next = cur.next.left             cur = cur.next        }
    }