206. 反转链表

/**
     * 迭代方法
     * 1 -> 2 -> 3 -> 4 -> null
     * null <- 1 <- 2 <- 3 <- 4
     *
     * @param head
     * @return
     */
    public static ListNode reverseListIterative(ListNode head) {
        ListNode prev = null; //前指针节点
        ListNode curr = head; //当前指针节点
        //每次循环，都将当前节点指向它前面的节点，然后当前节点和前节点后移
        while (curr != null) {
            ListNode nextTemp = curr.next; //临时节点，暂存当前节点的下一节点，用于后移
            curr.next = prev; //将当前节点指向它前面的节点
            prev = curr; //前指针后移
            curr = nextTemp; //当前指针后移
        }
        return prev;
    }