python实验一 简单的递归应用

题目：

题解：

char* getPermutation(int n, int k) {
    int factorial[n];
    factorial[0] = 1;
    for (int i = 1; i < n; ++i) {
        factorial[i] = factorial[i - 1] * i;
    }

    --k;
    char* ans = malloc(n + 1);
    ans[n] = '\0';
    int valid[n + 1];
    for (int i = 0; i <= n; ++i) {
        valid[i] = 1;
    }
    for (int i = 1; i <= n; ++i) {
        int order = k / factorial[n - i] + 1;
        for (int j = 1; j <= n; ++j) {
            order -= valid[j];
            if (!order) {
                ans[i - 1] = j + '0';
                valid[j] = 0;
                break;
            }
        }
        k %= factorial[n - i];
    }
    return ans;
}