PROB：zerosum

题目来自 USACO
题目翻译见 nocow

题目很简单，自己都说了让我们暴搜。

运算符最多 8 个，每个 3 种，果然大家都是 0.000 sec 就 AC 了。

我这边也写得丑丑的。递归搜索（注意下 ascii 码顺序）。对表达式进行解析计算（我也不知道我怎么写的这一段，竟然就通过了），记录下左边的值、操作符、右边的值，如果下一个操作符是空格，就给右边添一位，是加减就把上次操作做了，记录新的操作符和右边。

/*
ID: 
LANG: C++
TASK: zerosum
*/

#include <cstdio>
#include <cstdlib>

int N;
FILE *fout;
char operations[8];

int parse(){
    char op = '+';
    int pre = 0, post = 1;
    for(int n = 2; n <= N; n ++){
        if(operations[n - 2] == ' '){
            post = post * 10 + n;
        }else{
            pre = (op == '+'? pre + post: pre - post);
            op = operations[n - 2];
            post = n;
        }   
    }
    pre = (op == '+'? pre + post: pre - post);
    return pre;
}

void search(int i){
    if(i == N - 1){
        if(parse() == 0){
            for(int i = 1; i < N; i ++)
                fprintf(fout, "%d%c", i, operations[i - 1]);
            fprintf(fout, "%d\n", N);
        }
        return;
    }
    operations[i] = ' '; search(i + 1);
    operations[i] = '+'; search(i + 1);
    operations[i] = '-'; search(i + 1);
}

int main(){
    FILE *fin = fopen("zerosum.in", "r");
    fout = fopen("zerosum.out", "w");

    fscanf(fin, "%d", &N);

    search(0);

    return 0;
}

第一次提交 AC：

Compiling...
Compile: OK

Executing...
   Test 1: TEST OK [0.000 secs, 4176 KB]
   Test 2: TEST OK [0.000 secs, 4176 KB]
   Test 3: TEST OK [0.000 secs, 4176 KB]
   Test 4: TEST OK [0.000 secs, 4176 KB]
   Test 5: TEST OK [0.000 secs, 4176 KB]
   Test 6: TEST OK [0.000 secs, 4176 KB]
   Test 7: TEST OK [0.000 secs, 4176 KB]

All tests OK.