367. Valid Perfect Square

n*n=1+2+3+...+(2n-1)

static int wing=[]()
{
    std::ios::sync_with_stdio(false);
    cin.tie(NULL);
    return 0;
}();

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