二分查找

// 记住：二分查找的前提是 已排序 的数组
function binarySearch(arr, target) {
    let len = arr.length;
    let low = 0;
    let high = len - 1;
    while (low <= high) {
        let mid = Math.floor((low + high)/2);
        if (arr[mid] > target) {
            high = mid - 1;
        } else if (arr[mid] < target) {
            low = mid + 1;
        } else if (arr[mid] === target) {
            return mid; // 找到了！
        }
    }
    return null; // 没找到～
}