367. 有效的完全平方数(java)

二分法：

class Solution {

public boolean isPerfectSquare(int num) {
  int left = 0,right = num;
  if(num==1)
  {
      return true;
  }
  while(left<right)
  {
      int mid = ((right-left)/2)+left;
      long x = (long)mid*mid;
      if(num<x)
      {
          right=mid;
      }else if(num>x)
        {
            left=mid+1;
        }else
        {
            return true;
        }
  }
  return false;

}

}
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暴力解法：

int n = 1;

while (num / n >= n) {
    if (num % n==0 && num/n == n) {
        return true;
    }
    n++;
}
return false;

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牛顿迭代：

class Solution {

public boolean isPerfectSquare(int num) {
    double x = num;
    while(x * x - num > 0.00000001){
        x = (num/x + x)/2;
    }
    int y = (int)x;
    return y*y == num;
}

}