二分查找

二分查找：实现中采用左闭右闭区间。在findPos中，如果要查找的数在数组中有重复值，则返回的是最靠近数组中间位置的值。在findPosFirst中，如果有重复值返回位置最前的值。在findPosLast中，如果有重复值返回位置最后的值。

public class Bsearch {public Bsearch(){}public static int[] array = new int[]{};public static void search(int t){int res = findPos(t,0,array.length-1);if(res==-1)System.out.println("Find the right value:"+"\t"+"Can not find "+t+" in the array!");elseSystem.out.println("Find the right value:"+"\t"+t+" is the "+(res+1)+" element in the array!");res = findPosFirst(t,0,array.length-1);if(res==-1)System.out.println("Find the first right value:"+"\t"+"Can not find "+t+" in the array!");elseSystem.out.println("Find the first right value:"+"\t"+t+" is the "+(res+1)+" element in the array!");res = findPosLast(t,0,array.length-1);if(res==-1)System.out.println("Find the last right value:"+"\t"+"Can not find "+t+" in the array!");elseSystem.out.println("Find the last right value:"+"\t"+t+" is the "+(res+1)+" element in the array!");}public static int findPos(int t, int start, int end){if(start>end) return -1;int l = start;int h = end;int mid = (l+h)/2;if(array[mid]<t){l = mid+1;return findPos(t,l,h);}else if(array[mid]==t){return mid;}else{h = mid-1;return findPos(t,l,h);}}public static int findPosFirst(int t, int start, int end){int pos = -1;int mid;while(start<=end){mid = (start+end)/2;if(array[mid]>t) end = mid-1;else if(array[mid]<t) start = mid+1;else{pos = mid;end = mid-1;}}return pos;}public static int findPosLast(int t, int start, int end){int pos=-1;int mid;while(start<=end){mid = (start+end)/2;if(array[mid]>t) end = mid-1;else if(array[mid]<t) start = mid+1;else {pos = mid;start = mid+1;}}return pos;}public static void main(String[] args){array = new int[]{1,2,2,2,2};//array = new int[]{1,2,3,4,5};int t = 2;search(t);}}