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Cogito ergo sum

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leetcode 148 Solution

发表于 更新于 分类于 180 1 分钟

代码解析

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package com.demo.s148;

/**
 * 排序链表
 * 给你链表的头结点 head ，请将其按 升序 排列并返回 排序后的链表 。
 */
class ListNode {
    int val;
    ListNode next;
    ListNode() {}
    ListNode(int val) { this.val = val; }
    ListNode(int val, ListNode next) { this.val = val; this.next = next; }
}
public class Solution {
    public ListNode sortList(ListNode head) {
        return sortList(head, null);
    }

    public ListNode sortList(ListNode head, ListNode tail) {
        if(head == null) {
            return head;
        }
        if(head.next == tail) {
            head.next = null;
            return head;
        }

        ListNode fast = head;
        ListNode slow = head;
        while(fast != tail) {
            fast  = fast.next;
            slow  = slow.next;
            if(fast != tail) {
                fast = fast.next;
            }
        }
        ListNode mid = slow;
        ListNode m = sortList(head, mid);
        ListNode n = sortList(mid, tail);
        return mergeList(m, n);
    }
    public ListNode mergeList(ListNode head, ListNode tail) {
        ListNode dum = new ListNode(0);
        ListNode tmp1 = head;
        ListNode tmp2 = tail;
        ListNode tmp = dum;
        while(tmp1 != null && tmp2 != null) {
            if(tmp1.val < tmp2.val) {
                tmp.next = tmp1;
                tmp1 = tmp1.next;
            } else {
                tmp.next = tmp2;
                tmp2 = tmp2.next;
            }
            tmp = tmp.next;
        }
        if(tmp1 != null) {
            tmp.next = tmp1;
        }
        if(tmp2 != null) {
            tmp.next = tmp2;
        }
        return dum.next;
    }
}