经典面试题：请手写一个二分查找法
二分查找又称折半查找，它是一种效率较高的查找方法。
1.二分查找要求：
（1）必须采用顺序存储结构
（2）.必须按关键字大小有序排列
2.二分查找法原理：
3.二分查找Jdk8源码：
// Like public version, but without range checks.private static int binarySearch(int[] a, int fromIndex, int toIndex,int key) {int low = fromIndex;int high = toIndex - 1;while (low <= high) {int mid = (low + high) >>> 1;int midVal = a[mid];if (midVal < key)low = mid + 1;else if (midVal > key)high = mid - 1;elsereturn mid; // key found}return -(low + 1); // key not found.}
4.递归实现二分查找：
//递归实现二分查找private static int binarySearchDiGui(int[] arr, int data, int beginIndex, int endIndex) {if (arr == null) {return -1;}int midIndex = (beginIndex + endIndex) >>> 1;if (data < arr[beginIndex] || data > arr[endIndex] || beginIndex > endIndex) {return -1;}if (data > arr[midIndex]) {//递归，如果查找的数据大于中值，则以中值+1为起始索引查找return binarySearchDiGui(arr, data, midIndex + 1, endIndex);}if (data < arr[midIndex]) {//递归，如果查找的数据大于中值，则以中值+1为起始索引查找return binarySearchDiGui(arr, data, beginIndex, midIndex - 1);} else {return midIndex;}}