How to use binary method to find the position of array elements in Java

1. Explanation of the dichotomy method

The core idea of ​​the dichotomy method is the movement of the index, and the search speed increases geometrically.

Binary search method, returns the index of the array element found, if not found, returns -1

2, Example

Binary search positioning parameters The position of the value in the array

Scene description:

Find its subscript range in the array based on a parameter value, for example: 2 is in the array {0, 1, 3, 5} The interval is {1, 2}

package com.study.collection;
 
import java.util.Arrays;
 
/**
 * @auth zhangmj
 * @date 2019/2/12 9:14
 */
public class ExampleList<T> {
 
    public static void main(String[] args) {
        int[] intArray = {0,1,2,10,15,20,25,29,31,36,39,40,42,43,46,50,55,60,63,66,70};
        int num =2;
        int[] resultArray = getPostionByTwoPoint(intArray, num);
        System.out.println(Arrays.toString(resultArray));
    }
 
    private static int[] getPostionByTwoPoint(int[] intArray, int num) {
        // 判断
        if(intArray == null || intArray.length == 0){
            throw new RuntimeException("数组不能为空");
        }
        // 定义最小和区间
        if(intArray[0] > num || intArray[intArray.length - 1] < num){
            throw new RuntimeException("不在数组范围之内");
        }
 
        int middle = 0;
        int low = 0;
        int high = intArray.length - 1;
        // 定义首尾特殊的情况
        if(intArray[low] == num){
            int[] resultArray = {low, low};
            return resultArray;
        }else if(intArray[high] == num){
            int[] resultArray = {high, high};
            return resultArray;
        }
        int i = 1;
        // 数在中间的情况
        while(low < high){
            System.out.println("查找第 " + i + " 次");
            middle = (low + high + 1)/2;
            if(intArray[middle] == num){
                int[] resultArray = {middle, middle};
                return resultArray;
            }else if(intArray[middle] > num){
                // num 在 low 和 middle 之间
                int previous = middle - 1;
                if(previous > low  && intArray[previous] < num){
                    int[] resultArray = {previous, middle};
                    return resultArray;
                }
                high = middle;
            }else if(intArray[middle] < num){
                int latter = middle + 1;
                if(latter < high  && intArray[latter] > num){
                    int[] resultArray = {middle, latter};
                    return resultArray;
                }
                low = middle;
            }
            i++;
        }
        throw new RuntimeException("定位异常");
    }
}

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