歸併排序求逆序對數

標籤：歸併排序   逆序對數   

http://poj.org/problem?id=2299

給出n個數，每次只能交換兩個相鄰的數，問使得n個數有序最少需要交換多少次。

歸併排序的模板，重在理解，小白書p144.

#include <stdio.h>#include <map>#include <set>#include <stack>#include <queue>#include <vector>#include <cmath>#include <stdio.h>#include <stdlib.h>#include <cstring>#include <iostream>#include <limits.h>#include <algorithm>#define LL long long#define _LL __int64using namespace std;const int maxn = 500010;LL cnt;int n,a[maxn],t[maxn];void merge_sort(int *a, int x, int y, int *t){if(y-x > 1){int m = x + (y-x)/2;int p = x,q = m,i = x;merge_sort(a,x,m,t);merge_sort(a,m,y,t);while(p < m || q < y){if(q >= y || (p < m && a[p] <= a[q]))t[i++] = a[p++];else{t[i++] = a[q++];cnt += m-p;}}for(i = x; i < y; i++)a[i] = t[i];}}int main(){while(~scanf("%d",&n)&&n){for(int i = 0; i < n; i++)scanf("%d",&a[i]);cnt = 0;merge_sort(a,0,n,t);printf("%lld\n",cnt);}return 0;}

http://acm.hdu.edu.cn/showproblem.php?pid=4911

每次只能交換兩個相鄰的數，最多交換K次，問最後的最少的逆序對數是多少

#include <stdio.h>#include <map>#include <set>#include <stack>#include <queue>#include <vector>#include <cmath>#include <stdio.h>#include <stdlib.h>#include <cstring>#include <iostream>#include <limits.h>#include <algorithm>#define LL long long#define _LL __int64using namespace std;const int maxn = 500010;_LL cnt,k;int n,a[maxn],t[maxn];void merge_sort(int *a, int x, int y, int *t){if(y-x > 1){int m = x + (y-x)/2;int p = x,q = m,i = x;merge_sort(a,x,m,t);merge_sort(a,m,y,t);while(p < m || q < y){if(q >= y || (p < m && a[p] <= a[q]))t[i++] = a[p++];else{t[i++] = a[q++];cnt += m-p;}}for(i = x; i < y; i++)a[i] = t[i];}}int main(){while(~scanf("%d %I64d",&n,&k)){for(int i = 0; i < n; i++)scanf("%d",&a[i]);cnt = 0;merge_sort(a,0,n,t);if(k >= cnt)printf("0\n");elseprintf("%I64d\n",cnt-k);}return 0;}