删除倒数第n个节点

找到倒数第k个节点和倒数第k个的前一个节点

res = new Node();

pre = res;

cur = head;

fast = head;

先把快指针向后移动n次

while(n > 0){fast = fast.next;n--}

再把快指针和慢指针一起向后移动，直到快指针为空

while(fast != null){fast = fast.next;　　pre = cur;　　cur = cur.next}

此时fast为空，cur为倒数第n个节点，删除节点

pre.next = cur.next;

return res.next;

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