leetcode day7

这道题弄的心好累。。

 【Reverse Linked List】206

描述：

Reverse a singly linked list.

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Hint:

A linked list can be reversed either iteratively or recursively. Could you implement both?

思路：

增加一个全局节点newHead，用来指向新的头部，用用一个临时节点next来遍历整个链表，用来当旧的头部head，例如链表1-2-3-4，第一遍循环，newhead指向1，指向null，head是2,指向剩余的链表，2-3-4，然后第二遍循环，newhead指向2-1，head指向剩余的链表，3-4。。。以此类推，直到循环结束。

// iterative solution

public ListNode reverseList(ListNode head) {
    ListNode newHead = null;
    while(head != null){
        ListNode next = head.next;
        head.next = newHead;
        newHead = head;
        head = next;
    }
    return newHead;
}

还有递归的算法：

// recursive solution
public ListNode reverseList(ListNode head) {
    return reverseListInt(head, null);
}

public ListNode reverseListInt(ListNode head, ListNode newHead) {
    if(head == null)
        return newHead;
    ListNode next = head.next;
    head.next = newHead;
    return reverseListInt(next, head);
}