http://www.lydsy.com/JudgeOnline/problem.php?id=1653
看了题解才会的。。T_T
我们直接枚举每一种情况（这里用next_permutation，全排列）
然后判断是否符合情况（累加判断）
#include <cstdio>
#include <cstring>
#include <cmath>
#include <string>
#include <iostream>
#include <algorithm>
#include <queue>
using namespace std;
#define rep(i, n) for(int i=0; i<(n); ++i)
#define for1(i,a,n) for(int i=(a);i<=(n);++i)
#define for2(i,a,n) for(int i=(a);i<(n);++i)
#define for3(i,a,n) for(int i=(a);i>=(n);--i)
#define for4(i,a,n) for(int i=(a);i>(n);--i)
#define CC(i,a) memset(i,a,sizeof(i))
#define read(a) a=getint()
#define print(a) printf("%d", a)
#define dbg(x) cout << #x << " = " << x << endl
#define printarr(a, n, m) rep(aaa, n) { rep(bbb, m) cout << a[aaa][bbb]; cout << endl; }
inline const int getint() { int r=0, k=1; char c=getchar(); for(; c<'0'||c>'9'; c=getchar()) if(c=='-') k=-1; for(; c>='0'&&c<='9'; c=getchar()) r=r*10+c-'0'; return k*r; }
inline const int max(const int &a, const int &b) { return a>b?a:b; }
inline const int min(const int &a, const int &b) { return a<b?a:b; } int n, m;
int a[15], f[15];
int main() {
read(n); read(m);
for1(i, 1, n) a[i]=i;
do {
memcpy(f, a, sizeof(a));
for1(i, 1, n-1) for3(j, i, 1) f[j]+=f[j+1];
if(f[1]==m) {
printf("%d", a[1]);
for1(i, 2, n) printf(" %d", a[i]);
puts("");
break;
}
}while(next_permutation(a+1, a+1+n));
return 0;
}
Description
FJ and his cows enjoy playing a mental game. They write down the numbers from 1 to N (1 <= N <= 10) in a certain order and then sum adjacent numbers to produce a new list with one fewer number. They repeat this until only a single number is left. For example, one instance of the game (when N=4) might go like this: 3 1 2 4 4 3 6 7 9 16 Behind FJ's back, the cows have started playing a more difficult game, in which they try to determine the starting sequence from only the final total and the number N. Unfortunately, the game is a bit above FJ's mental arithmetic capabilities. Write a program to help FJ play the game and keep up with the cows.
Input
* Line 1: Two space-separated integers: N and the final sum.
Output
* Line 1: An ordering of the integers 1..N that leads to the given sum. If there are multiple solutions, choose the one that is lexicographically least, i.e., that puts smaller numbers first.
Sample Input
4 16
Sample Output
3 1 2 4
OUTPUT DETAILS:
There are other possible sequences, such as 3 2 1 4, but 3 1 2 4
is the lexicographically smallest.