Adjacent Swaps

Adjacent Swaps

https://atcoder.jp/contests/abc250/tasks/abc250_c

 

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Solution

这是一个典型的利用双向链表来提供性能的问题。

利用双向链表，需要注意：

（1）添加 表头 和 表尾 节点。

（2）抽象出交换相邻节点，需要用到的 prev 和 next_next节点。

#include <bits/stdc++.h>
#include <vector>
#include <algorithm>
#include <deque>
#include <queue>
#include <string>
#include <set>
using namespace std;


/*
https://atcoder.jp/contests/abc250/tasks/abc250_c
*/

struct node {
    int prev;
    int next;
};

const int limit = 200010;

struct node balls[limit] = {
    {
        0, 0
    }
};

int main() {
    int n, q;

    cin >> n >> q;

//    cout << to_string(n) << endl;

    // initialize n ball node prev and next
    // normally all previous and next pointer value must be in range [1, n]
    // but for first node, its previous pointer is 0, called head
    // and for last node, its next pointer is n+1, called tail
    int head_index = 0;
    int tail_index = n + 1;
    balls[head_index].next = 1;
    balls[tail_index].prev = n;

    for (int i = 1; i <= n; i++) {
        balls[i].prev = i - 1;
        balls[i].next = i + 1;
    }

    for (int i = 0; i < q; i++) {
        int xi;
        cin >> xi;

        // need to swap curr_index and next_index
        int curr_index = xi;
        int next_index = balls[curr_index].next;

        int prev_index = 0;
        int next_next_index = 0;

        // current ball is the last ball,
        // set curr_index as n-2, next_index as n-1
        if (next_index == tail_index) {
            int prev_curr_index = balls[curr_index].prev;
            next_index = curr_index;
            curr_index = prev_curr_index;
        }

        // find two helping index
        prev_index = balls[curr_index].prev;
        next_next_index = balls[next_index].next;

        //swap curr and next
        balls[prev_index].next = next_index;
        balls[next_next_index].prev = curr_index;

        balls[curr_index].prev = next_index;
        balls[curr_index].next = next_next_index;

        balls[next_index].prev = prev_index;
        balls[next_index].next = curr_index;
    }

    // print
    int curr_index = balls[head_index].next;
    while (curr_index != tail_index) {
        cout << curr_index;
        curr_index = balls[curr_index].next;

        if (curr_index != tail_index) {
            cout << " ";
        }
    }
}