*/-->

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dfs：部分和问题

给定整数a₁, a₂ … a_{n，判断是否从中可以选出若干数，使他们的和恰好为k}

注意：

每个数只能选择一次，当然也可以不选

// CreateTime: 2015-04-07 22:55:39

#include <iostream>

using namespace std;

int n;
int a[100005];
int k;
int ok;

void dfs(int step, int sum) {
    if (n == step) {
        if (sum == k) {
            ok = 1;
        }
        return;
    }
    dfs(step+1, sum);
    dfs(step+1, sum+a[step]);
}

int main(void) {
    cin >> n;
    for (int i = 0; i < n; i++) {
        cin >> a[i];
    }
    cin >> k;

    ok = 0;
    dfs(0, 0);

    if (ok) {
        cout << "Yes" << endl;
    } else {
        cout << "No" << endl;
    }

    return 0;
}
// 4 1 2 4 7 13
// 4 1 2 4 7 15

还可以写的更美观点

// CreateTime: 2015-04-07 22:55:39

#include <iostream>

using namespace std;

int n;
int a[100005];
int k;
int ok;

int dfs(int step, int sum) {
    if (n == step) {
        if (sum == k) {
            return 1;
        } else {
            return 0;
        }
    }
    if(dfs(step+1, sum)) {
        return 1;
    }
    if(dfs(step+1, sum+a[step])) {
        return 1;
    }
}

int main(void) {
    cin >> n;
    for (int i = 0; i < n; i++) {
        cin >> a[i];
    }
    cin >> k;

    ok = 0;
    if(dfs(0, 0)) {
        cout << "Yes" << endl;
    } else {
        cout << "No" << endl;
    }

    return 0;
}
// 4 1 2 4 7 13
// 4 1 2 4 7 15