Leetcode effective perfect square number c++

Effective perfect square

Given a positive integer num, write a function that returns True if num is a perfect square number, otherwise returns False.

Note: Do not use any built-in library functions, such as sqrt.

Example 1:

输入：16
输出：True

Example 2:

输入：14
输出：False

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Solution 1: Incremental judgment

public:
    bool isPerfectSquare(int num) {
        int i=1;
        long n=i*i;
        while(n<=num)
        {
            if(n==num)
                return true;
            else 
            {
                i++;
                n=pow(i,2);
            }
        }
        return false;
    }
};

Solution 2: Dichotomy

class Solution {
public:
    bool isPerfectSquare(int num) {
        int start=1;
        int end=num;
        int mid=start+(end-start)/2;
        while(start<=end)
        {
            if(pow(mid,2)>num)
            {
                end=mid-1;
            }
            else if(pow(mid,2)<num)
            {
                start=mid+1;
            }
            else return true;
            mid=(end-start)/2+start;
        }
        return false;
    }
};

Solution 3: Formula method

Use 1+3+5+7+9+…+(2n-1)=n^2, that is, the perfect square number must be the sum of the first n consecutive odd numbers

class Solution {
public:
    bool isPerfectSquare(int num) {
        int i=1;
        while(num>0)
        {
            num-=i;
            i+=2;
        }
        return num==0;
    }
};