69. x 的平方根（算法200题-81）

var mySqrt = function(x) {
    // X = 8 , [1,2,3,4,5,6,7,8]
    let left = 0,
        right = x,
        center = 0
    while(left <= right) {
        center = left + ((right - left) >> 1)
        if(center * center < x) {
            left = center + 1
        } else if(center * center > x) {
            right = center - 1
        } else {
            return center
        }
    }
    return right
};