使序列有序的最少交换次数（minimum swaps）

交换相邻两数

如果只是交换相邻两数，那么最少交换次数为该序列的逆序数。

交换任意两数

数字的总个数减去循环节的个数？？

A cycle is a set of elements, each of which is in the place of another.  So in example sequences { 2, 1, 4, 3}, there are two cycles: {1, 2} and {3, 4}.  1 is in the place where 2 needs to Go, and 2 is in the place where 1 needs to go 1, so they are a cycle; likewise with 3 and 4.

The sequences {3, 2, 1, 5, 6, 8, 4, 7 }also has two cycles: 3 is in the place of 1 which is in the place of 3, so {1, 3} is a cycle; 2 is in its proper place; and 4 is in the place of 7 which is in the place of 8 in place of 6 in place of 5 in place of 4, so {4, 7, 8, 6, 5} is a cycle.  There are seven elements out of place in two cycles, so five swaps are needed.

实现：

e.g. { 2, 3, 1, 5, 6, 4}

231564 -> 6 mismatch 
two cycles -> 123 and 456 
swap 1,2 then 2,3, then 4,5 then 5,6 -> 4 swaps to sort 
 
Probably the easiest algorithm would be to use a bitarray. Initialize it to 0, then start at the first 0.

Swap the number there to the right place and put a 1 there.

Continue until the current place holds the right number.

Then move on to the next 0 

有序列5，4，3，2，1。共5个数。

nums [0] [1] [2] [3] [4]

　　　　5 4 3 2 1

按升序排列之后为

nums1 [0] [1] [2] [3] [4]

     1 2 3 4  5 

5应该到1处，1应该到5处，形成了一个循环，所以可以将它们抽象成一个环，环内换序就可以了。(这种环称为循环节) 
如果把它们两个看成整体，对于整个序列来说它们占据了排好序后5，1应该在的位置，所以对于整个序列来说是有序的，
它们只是自身内部无序而已。
上例中3在原本就在的位置，可以看成一个元素的循环节。 
我们可以推断出有一个循环节，就可以少交换一次，因为n个元素的循环节，只需交换n-1次即可有序。 
那么对于整个序列来说，最少交换次数为 元素总数-循环节个数。

Example: 

231564 
000000 
-> swap 2,3 
321564 
010000 
-> swap 3,1 
123564 
111000 
-> continue at next 0; swap 5,6 
123654 
111010 
-> swap 6,4 
123456 
111111 
-> bitarray is all 1's, so we're done. 

#include "bits/stdc++.h"
using namespace std;
/*
 *  交换任意两数的本质是改变了元素位置，
 *  故建立元素与其目标状态应放置位置的映射关系
 */
int getMinSwaps(vector<int> &A)
{
    //  排序
    vector<int> B(A);
    sort(B.begin(), B.end());
    map<int, int> m;
    int len = (int)A.size();
    for (int i = 0; i < len; i++)
    {
        m[B[i]] = i;    //  建立每个元素与其应放位置的映射关系
    }

    int loops = 0;      //  循环节个数
    vector<bool> flag(len, false);
    //  找出循环节的个数
    for (int i = 0; i < len; i++)
    {
        if (!flag[i])
        {
            int j = i;
            while (!flag[j])
            {
                flag[j] = true;
                j = m[A[j]];    //  原序列中j位置的元素在有序序列中的位置
            }
            loops++;
        }
    }
    return len - loops;
}

vector<int> nums;

int main()
{
    nums.push_back(1);
    nums.push_back(2);
    nums.push_back(4);
    nums.push_back(3);
    nums.push_back(5);

    int res = getMinSwaps(nums);

    cout << res << '
';

    return 0;
}

交换任意区间

？？

#include "bits/stdc++.h"
using namespace std;
/*
 *  默认目标映射关系是 key 1 => val 1 …… key n => val n
 *  如果序列不是 1~n 可以通过 map 建立新的目标映射关系
 *  交换任意区间的本质是改变了元素的后继，故建立元素与其初始状态后继的映射关系
 */
const int MAXN = 30;

int n;
int vis[MAXN];
int A[MAXN], B[MAXN];

int getMinSwaps()
{
    memset(vis, 0, sizeof(vis));

    for (int i = 1; i <= n; i++)
    {
        B[A[i]] = A[i % n + 1];
    }
    for (int i = 1; i <= n; i++)
    {
        B[i] = (B[i] - 2 + n) % n + 1;
    }

    int cnt = n;
    for (int i = 1; i <= n; i++)
    {
        if (vis[i])
        {
            continue;
        }
        vis[i] = 1;
        cnt--;
        for (int j = B[i]; j != i; j = B[j])
        {
            vis[j] = 1;
        }
    }

    return cnt;
}

int main()
{
    cin >> n;
    for (int i = 1; i <= n; i++)
    {
        cin >> A[i];
    }

    int res = getMinSwaps();

    cout << res << '
';

    return 0;
}