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排序链表

2022/12/04

https://leetcode.cn/problems/sort-list/

示例​

示例一​

输入：head = [4,2,1,3]
输出：[1,2,3,4]

示例二​

输入：head = [-1,5,3,4,0]
输出：[-1,0,3,4,5]

示例三​

输入：head = []
输出：[]

题解参考​

自顶向下归并排序​

const merge = (head1, head2) => {
    const dummyHead = new ListNode(0);
    let temp = dummyHead, temp1 = head1, temp2 = head2;
    while (temp1 !== null && temp2 !== null) {
        if (temp1.val <= temp2.val) {
            temp.next = temp1;
            temp1 = temp1.next;
        } else {
            temp.next = temp2;
            temp2 = temp2.next;
        }
        temp = temp.next;
    }
    if (temp1 !== null) {
        temp.next = temp1;
    } else if (temp2 !== null) {
        temp.next = temp2;
    }
    return dummyHead.next;
}

const toSortList = (head, tail) => {
    if (head === null) {
        return head;
    }
    if (head.next === tail) {
        head.next = null;
        return head;
    }
    let slow = head, fast = head;
    while (fast !== tail) {
        slow = slow.next;
        fast = fast.next;
        if (fast !== tail) {
            fast = fast.next;
        }
    }
    const mid = slow;
    return merge(toSortList(head, mid), toSortList(mid, tail));
}

var sortList = function(head) {
    return toSortList(head, null);
};

自底向上归并排序​

const merge = (head1, head2) => {
    const dummyHead = new ListNode(0);
    let temp = dummyHead, temp1 = head1, temp2 = head2;
    while (temp1 !== null && temp2 !== null) {
        if (temp1.val <= temp2.val) {
            temp.next = temp1;
            temp1 = temp1.next;
        } else {
            temp.next = temp2;
            temp2 = temp2.next;
        }
        temp = temp.next;
    }
    if (temp1 !== null) {
        temp.next = temp1;
    } else if (temp2 !== null) {
        temp.next = temp2;
    }
    return dummyHead.next;
}

var sortList = function(head) {
    if (head === null) {
        return head;
    }
    let length = 0;
    let node = head;
    while (node !== null) {
        length++;
        node = node.next;
    }
    const dummyHead = new ListNode(0, head);
    for (let subLength = 1; subLength < length; subLength <<= 1) {
        let prev = dummyHead, curr = dummyHead.next;
        while (curr !== null) {
            let head1 = curr;
            for (let i = 1; i < subLength && curr.next !== null; i++) {
                curr = curr.next;
            }
            let head2 = curr.next;
            curr.next = null;
            curr = head2;
            for (let i = 1; i < subLength && curr != null && curr.next !== null; i++) {
                curr = curr.next;
            }
            let next = null;
            if (curr !== null) {
                next = curr.next;
                curr.next = null;
            }
            const merged = merge(head1, head2);
            prev.next = merged;
            while (prev.next !== null) {
                prev = prev.next;
            }
            curr = next;
        }
    }
    return dummyHead.next;
};