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// 求近似值
func sqrt(x float64) float64 {  
   l, r := float64(1), x  
   mid := (l + r) / 2  
   for mid-l > 0.000001 {  
      if mid*mid > x {  
         r = mid  
      } else {  
         l = mid  
      }  
      mid = (l + r) / 2  
   }  
   return mid  
}  

// 只能求整数部分
func mySqrt(x float64) float64 {  
   l, r := float64(0), x  
   ans := -1.0  
   for l <= r {  
      mid := l + (r-l)/2  
      if mid*mid > x {  
         r = mid - 1  
      } else {  
         ans = mid  
         l = mid + 1  
      }  
   }  
   return ans  
}