1. 暴力法：遍歷所有可能的完全平方數，判斷是否等于給定的數。

#include <stdio.h>

int isPerfectSquare(int num) {
    for (int i = 1; i * i <= num; i++) {
        if (i * i == num) {
            return 1;
        }
    }
    return 0;
}

int main() {
    int num;
    printf("Enter a number: ");
    scanf("%d", &num);

    if (isPerfectSquare(num)) {
        printf("%d is a perfect square.\n", num);
    } else {
        printf("%d is not a perfect square.\n", num);
    }

    return 0;
}

2. 二分查找法：利用二分查找的思想，在范圍內查找給定數的平方根，判斷是否為完全平方數。

#include <stdio.h>

int isPerfectSquare(int num) {
    long left = 1, right = num;

    while (left <= right) {
        long mid = left + (right - left) / 2;
        long square = mid * mid;

        if (square == num) {
            return 1;
        } else if (square < num) {
            left = mid + 1;
        } else {
            right = mid - 1;
        }
    }

    return 0;
}

int main() {
    int num;
    printf("Enter a number: ");
    scanf("%d", &num);

    if (isPerfectSquare(num)) {
        printf("%d is a perfect square.\n", num);
    } else {
        printf("%d is not a perfect square.\n", num);
    }

    return 0;
}

這兩種算法都可以判斷一個數是否為完全平方數，第二種算法的效率更高一些。