69. x 的平方根

https://leetcode-cn.com/problems/sqrtx/

func mySqrt(_ x: Int) -> Int {
    if x < 2 {return x}
    var pivot = 0
    var left = 2
    var right = x / 2
    while left <= right {
        pivot = left + (right - left) / 2
        let sum = pivot * pivot
        if sum > x {
            right = pivot - 1
        } else if sum < x {
            left = pivot + 1
        } else {
            return pivot
        }
    }
    return right
}