二分查找

var search = function(nums, target) {
    let start = 0
    let end = nums.length - 1
    while(start <= end) {
        let mid = Math.floor((start + end)/2)
        if (target < nums[mid]) {
            end = mid - 1
        } else if (target > nums[mid]) {
            start = mid + 1
        } else {
            return mid
        }
    }
    return -1
};