数组&链表 快速排序

关于思想以及代码别的博客太多了，不做冗余的介绍。

对于数组

public void sort(int[] array, int beg, int end) {    int index = partition(array, beg, end); //哨兵是也    sort(array, beg, index - 1); //哨兵左    sort(array, index + 1, end); //哨兵右分别递归sort方法    /* 要知道上面的递归写法都是针对整个数组的，对于寻找第k小的数得这么写 */    /*while (beg < end) {        int index = partition(nums, beg , end);        if (index > k - 1)            end = index - 1;        else if (index < k - 1)            beg = index + 1;        else            break;    }    return nums[k - 1];*/}private int partition(int[] array, beg, end) {    int key = array[beg]; //beg做关键字    while (beg < end) {        while (array[end] >= key && beg < end) {            end--;        }        array[beg] = array[end];        while (array[beg] <= key && beg < end) {            beg++;        }        array[end] = array[beg];    }    array[end] = key;    return end;}

对于链表

public void sort(ListNode head, ListNode end) {    if (head != end) {        ListNode mid = partition(head, end);        sort(head, mid);        sort(mid.next, end);    }}public ListNode partition(ListNode head, ListNode end) {    if (head.next == null)        return head;    ListNode p = head;    ListNode q = head.next;    int key = p.val;    while (q != end) { //注意这里跟数组排序有点不一样        if (q.val < key) {            p = p.next; //p作为慢指针想找比key小的跟他交换，好狡猾            swap_val(p, q);        }        q = q.next;    }    swap_val(head, p); //最后把head扔到哨兵的位置    return p; //返回哨兵}

关于链表

关于链表还想说一件事——归并排序，顺便总结一下

public ListNode sortList(ListNode head) {    if (head == null || head.next == null)        return head;    ListNode mid = getMid(head); //得到中间的函数，很关键，很机智    ListNode rHead = mid.next;    mid.next = null; //把尾巴清空    return merge(sortList(head), sortList(rHead));//左半边右半边递归}private ListNode getMid(ListNode head) {    if (head.next.next == null)        return head;    ListNode slow, fast;    slow = fast = head; //快慢指针，快指针越界了就说明慢指针到正中间    while (fast.next != null && fast.next.next != null) {/*注意这里是next的next*/        slow = slow.next;        fast = fast.next.next;    }    return slow;}private ListNode merge(ListNode head, ListNode rHead) {        ListNode dummy = new ListNode(0);        ListNode cur = dummy;        /* 这里就相当于合并两个已排序的链表，是不是很机智？ */        while (head != null && rHead != null) {            if (head.val < rHead.val) {                cur.next = head;                head = head.next;            } else {                cur.next = rHead;                rHead = rHead.next;            }            cur = cur.next;        }        if (head == null) {            cur.next = rHead;        } else            cur.next = head;        return dummy.next;    }