LeetCode – 69. Sqrt(x)

題目連結: https://leetcode.com/problems/sqrtx/

題目:

Given a non-negative integer x, compute and return the square root of x.

Since the return type is an integer, the decimal digits are truncated, and only the integer part of the result is returned.

Note: You are not allowed to use any built-in exponent function or operator, such as pow(x, 0.5) or x ** 0.5.

解法一: (Time complexity: O(n))

   // test case:
   // x: 0
   // x: 1
   // x: 4
   // x: 8
   func mySqrt(_ x: Int) -> Int {
        if x == 0 { return 0 }
        var result = 0
        while (result + 1) * (result + 1) <= x {
        	result += 1
        }
        return result
    }

解法二: (Time complexity: O(logn))

Use Binary search

   // test case:
   // x: 0
   // x: 1
   // x: 4
   // x: 8
   func mySqrt(_ x: Int) -> Int {
        var start = 0
        var end = x
        while start < end {
            var mid = (start + end) / 2
        	if mid * mid == x {
        		start = mid
        		break
        	} else if mid * mid > x {
        		end = mid - 1
        	} else {
        		start = mid + 1
        	}
        }
        return start * start > x ? start - 1 : start
    }