算法设计课程复习(4)-分治算法

分治算法

求逆序对数

AC代码：

//用int会出错，改成用long long
#include <iostream>
#include <cstdio>
#include <vector>
#define Max 500005
using namespace std;

long long merge(int input[], int start, int mid, int end) {
    vector<int> temp_input;
    int m1 = start, m2 = mid+1;
    long long count = 0;
    while(m1 <= mid && m2 <= end) {
        if(input[m1] > input[m2]) {
            count += mid-m1+1;
            temp_input.push_back(input[m2]);
            m2++;
        }
        else {
            temp_input.push_back(input[m1]);
            m1++;
        }
    }
    while(m1 <= mid) {
        temp_input.push_back(input[m1]);
        m1++;
    }
    while(m2 <= end) {
        temp_input.push_back(input[m2]);
        m2++;
    }
    int k = 0;
    for(int i=start; i<=end; i++) {
        input[i] = temp_input[k++];
    }
    return count;
}

long long solve(int input[], int start, int end) {
    if(start >= end) return 0;
    int mid = (start+end)/2;
    long long left = solve(input, start, mid);
    long long right = solve(input, mid+1, end);
    long long left_right = merge(input, start, mid, end);
    return left+right+left_right;
}

int main(int argc, const char * argv[]) {
    // insert code here...
    int n, input[Max];
    while(scanf("%d", &n) && n!=0) {
        for(int i=0; i<n; i++) {
            scanf("%d", &input[i]);
        }
        long long result = solve(input, 0, n-1);
        cout<<result<<endl;
    }
    return 0;
}