Problem:
Score : 200 points

Problem Statement

You are given a positive integer X.
Find the largest perfect power that is at most X.
Here, a perfect power is an integer that can be represented as b^p, where b is an integer not less than 1 and p is an integer not less than 2.

Constraints

1 ≤ X ≤ 1000

X is an integer.

Input

Input is given from Standard Input in the following format:

X

Output

Print the largest perfect power that is at most X.

Sample Input 1

10

Sample Output 1

9

There are four perfect powers that are at most 10: 1, 4, 8 and 9.
We should print the largest among them, 9.

Sample Input 2

1

Sample Output 2

1

Sample Input 3

999

Sample Output 3

961

Solution in C (GCC 5.4.1) (judge verdict: Wrong Answer)

#include <stdio.h>

int main(int argc, const char * argv[]) {
    // insert code here...
    
    int x = 0;
    //2 3 5 7 11 13 17 19 23 29 31
    int value[11] = {2,3,5,7,11,13,17,19,23,29,31};
    int flag = 0;
    int maxValue = 0;
    
//    scanf("%d",&x);
    if( scanf("%d", &x) == 1)
    
    if (x == 1) {//1の場合は返す
        printf("1\n");
        return 0;
    }
    if (x == 2) {//2の場合は返す
        printf("1\n");
        return 0;
    }
    if (x == 3) {//3の場合は返す
        printf("1\n");
        return 0;
    }
    
    for (int i = 0; i < 11; i++) {//素数のどこまでかを把握する処理
        if(value[i] <= x){
            flag++;
        }
//        printf("flag=%d",flag);
    }
    if(flag > 1){
        flag = flag - 1;
    }
    
    while (flag != -1) {
        int number = value[flag];
//        printf("flag = %d\n",flag);
//        printf("number = %d\n",number);
        while (value[flag]*number <= x) {
            if (maxValue <= value[flag]*number) {
//                printf("value[flag] = %d\n",value[flag]);
//                printf("number = %d\n",number);
                maxValue = value[flag]*number;//上書き
//                printf("maxValue = %d\n",maxValue);
            }
            number *= value[flag];
        }
        flag--;
//        printf("flag = %d",flag);
    }
    
    printf("%d\n",maxValue);
    
    
    
    return 0;
}