Android_Java--二分法查找

二分法查找适用的条件是数据量较大，有序且不重复。
基本思路：
1、假如序列是升序的，给定一个查找的值x，找到序列a的中间位置y
2、如果x < a[y],则y - 1；
3、如果x > a[y],则y +1；
4、如果x = a[y],则直接返回相应位置；

package com.location.morgan.dichotomysearch;import android.support.v7.app.AppCompatActivity;import android.os.Bundle;import android.support.v7.view.menu.ActionMenuItemView;import android.util.Log;import android.view.View;public class MainActivity extends AppCompatActivity {    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        final int[] dichotomyArray = new int[]{          1,3,5,7,9,11,13,15,17,19,21        };        final int dichotomyKey = 19;        findViewById(R.id.bt).setOnClickListener(new View.OnClickListener() {            @Override            public void onClick(View v) {                int index = dichotomySearch(dichotomyArray,dichotomyKey);                Log.d("zzc","index is:"+index);            }        });    }    private int dichotomySearch(int[] dichotomyArray, int dichotomyKey) {        int start = 0;        int end = dichotomyArray.length - 1;        while(start <= end){            int middle = (start + end)/2;            if(dichotomyKey < dichotomyArray[middle]){                end = middle - 1;            }else if(dichotomyKey > dichotomyArray[middle]){                start = middle + 1;            }else{                return middle;            }        }        return -1;    }}