STL之 next_permutation函数{（全排列）按字典序！！！！！}

next_permutation函数

今天做题时，用到了这个函数，亮瞎我的双眼~~~
先贴出来题目！！！！！！

Description

You are to write a program that has to generate all possible words from a given set of letters.

Example: Given the word "abc", your program should - by exploring all different combination of the three letters - output the words "abc", "acb", "bac", "bca", "cab" and "cba".

In the word taken from the input file, some letters may appear more than once. For a given word, your program should not produce the same word more than once, and the words should be output in alphabetically ascending order.

Input

The input consists of several words. The first line contains a number giving the number of words to follow. Each following line contains one word. A word consists of uppercase or lowercase letters from A to Z. Uppercase and lowercase letters are to be considered
different. The length of each word is less than 13.

Output

For each word in the input, the output should contain all different words that can be generated with the letters of the given word. The words generated from the same input word should be output in alphabetically ascending order. An upper case letter goes before
the corresponding lower case letter.

Sample Input

3
aAb
abc
acba

Sample Output

Aab
Aba
aAb
abA
bAa
baA
abc
acb
bac
bca
cab
cba
aabc
aacb
abac
abca
acab
acba
baac
baca
bcaa
caab
caba
cbaa

Hint

An upper case letter goes before the corresponding lower case letter.

So the right order of letters is 'A'<'a'<'B'<'b'<...<'Z'<'z'.

题目意思大致就是说给你一行字符串，求它的全排列(按字典序)，但是要按照A < a < B < b<..........
刚看到题目，没多少思路，没想到STL有这么一个函数，先看看AC代码。

#include<stdio.h>
#include <ctype.h>
#include<string.h>
#include<math.h>
#include<algorithm>
using namespace std;
bool cmp(char a,char b) //'A'<'a'<'B'<'b'<...<'Z'<'z'.
{
if(tolower(a) != tolower(b))
return tolower(a)<tolower(b);
else
return a<b;
}
int main()
{
int n,l;
char a[15];
scanf("%d",&n);
while(n--)
{
scanf("%s",a);
l = strlen(a);
sort(a, a + l,cmp);
do
{
printf("%s\n",a);
}while(next_permutation(a, a + l, cmp));
}
return 0;
}

这里主要是说 next_permutation 函数的用法！！

next_permutation 的头文件#include<algorithm>
作用: 求全排列，按字典序排列！！！
（1）int类型的 的next_permutation

#include<stdio.h>
#include<string.h>
#include<math.h>
#include<algorithm>
using namespace std;
int main()
{
int a[3];
a[0] = 1;
a[1] = 2;
a[2] = 3;
do
{
printf("%d %d %d\n",a[0],a[1],a[2]);
}while(next_permutation(a,a + 3));
return 0;
}

(next_permutation(a,a + 3))这里的（a， a + 3)是地址，a是数组首地址，a + 3是第三个元素a[2]的地址，跟sort一样，意思是下一个排列组合，从a[0]到a[2]的排列。
(next_permutation(a,a + 2)) 也就是从a[0]到a[1]的排列。

上面代码输出
1 2 3

1 3 2

2 1 3

2 3 1

3 1 2

3 2 1

如果把a+3改成a + 2 就是
1 2 3

2 1 3

（2）char
类型的next_permutation

#include<stdio.h>
#include<string.h>
#include<math.h>
#include<algorithm>
using namespace std;
int main()
{
char a[200];
scanf("%s",a);
sort(a,a + strlen(a));
do
{
printf("%s\n",a);
}while(next_permutation(a,a + strlen(a)));
return 0;
}

意思跟int类型的含义一样，next_permutation（）括号里面是指针，也可以定义指针变量来指定排列。

#include<stdio.h>
#include<string.h>
#include<math.h>
#include<algorithm>
#include<iostream>
#include<string>
using namespace std;
int main()
{
string line;
while(cin>>line&&line!="#")
{
if(next_permutation(line.begin(),line.end())) //从当前输入位置开始
cout<<line<<endl;
}
}

int main()
{
string line;
while(cin>>line&&line!="#")
{
sort(line.begin(),line.end());//全排列
cout<<line<<endl;
while(next_permutation(line.begin(),line.end()))
cout<<line<<endl;
}
}

两个函数，一个是从当前排列，一个是全排列，跟平成一样
（4）自定义比较函数的全排列（代码见上面题目AC代码）
自定义比较函数跟构造sort自定义比较函数一模一样，用法也是一样
next_permutation(首地址，尾地址，构造的比较函数)。
上面AC代码用了tolower()；函数，这个函数的作用就是把大写变成小写。还有toupper()把小写转换成大写。
大小写转换函数的头文件都是#include <ctype.h>。
（5）类似的next_permutation还有与之完全相反的函数还有prev_permutation
作用是得到上一个排列。
代码可以验证。

#include<stdio.h>
#include<string.h>
#include<math.h>
#include<algorithm>
using namespace std;
int main()
{
int a[3];
a[0] = 3;
a[1] = 2;
a[2] = 1;
do
{
printf("%d %d %d\n",a[0],a[1],a[2]);
}while(prev_permutation(a,a + 3));
return 0;
}

输出为：

3 2 1

3 1 2

2 3 1

2 1 3

1 3 2

1 2 3