HDU 1258 Sum It Up(DFS)

Sum It Up

#include <bits/stdc++.h>
using namespace std;

const int maxn = 15;
int nums[maxn];
int ans[maxn];
int len;
bool ok;
int sum;
int n;
bool used[maxn];
bool cmp(int a, int b) {
    return a > b;
}

void dfs(int idx, int now) {
    if(idx <= n && now == sum) {//找到1个解
        if(!ok) {
            printf("Sums of %d:\n", sum);
        }

        ok = true;
        for(int i = 0; i <= len - 2; i++) {
            printf("%d+", ans[i]);
        }
        printf("%d\n", ans[len-1]);
        return;
    }

    if(idx > n || now > sum) {//不可能有解了
        return;
    }

    //开始选择
    for(int i = idx; i < n; ++i) {
        if(!used[i]) {
            if(i > 0 && nums[i] == nums[i-1] && !used[i-1]) {
                used[i] = false;
                continue;
            }

            if(now + nums[i] <= sum) {//可以选
                used[i] = true;//选它
                ans[len++] = nums[i];
                now += nums[i];
                dfs(i+1, now);

                used[i] = false;//不选它
                len--;
                now -= nums[i];

            } else {//不可以选
                used[i] = false;//不选它
            }
        }
    }
}

int main() {
    freopen("data.in", "r", stdin);

    while(scanf("%d %d", &sum, &n) == 2 && sum && n) {
        for(int i = 0; i < n; ++i) {
            scanf("%d", &nums[i]);
        }

        sort(nums, nums+n, cmp);
        ok = false;
        memset(used, false, sizeof(used));
        memset(ans, -1, sizeof(ans));
        len = 0;
        dfs(0, 0);
        if(!ok) {
            printf("Sums of %d:\n", sum);
            printf("NONE\n");
        }
    }

    return 0;
}