二分查找模版

二分查找模版

package com.zhang.reflection.面试.算法模版.二分查找模版;
public class 二分查找模版 {
    /**
     *  查找满足 x ≥ target 的下界的第一个元素下标，如果不存在返回数组的长度
     *  同理找上界，找下界，找特定值第一次出现的位置都可以，前提是数组有序或者部分有序
     */

    //找下界
    public static int LowerBound(int[] nums, int target) {
        int left=0;
        int right=nums.length-1;
        //跳出条件
        while(left<=right){
            int mid=left+(right-left)/2;
            if(nums[mid]>=target){
                right=mid-1;
            }else{
                left=mid+1;
            }
        }
        return left;
    }
    //找上界
    public static int UpperBound(int[] nums, int target) {
        int left=0;
        int right=nums.length-1;
        //跳出条件
        while(left<=right){
            int mid=left+(right-left)/2;
            if(nums[mid]>target){
                right=mid-1;
            }else{
                left=mid+1;
            }
        }
        return left-1;
    }
    //找该值第一次出现的位置
    public static int FirstBound(int[] nums, int target) {
        int left=0;
        int right=nums.length-1;
        //跳出条件
        while(left<=right){
            int mid=left+(right-left)/2;
            if(nums[mid]>=target){
                right=mid-1;
            }else{
                left=mid+1;
            }
        }
        if(left>=nums.length||nums[left]!=target){
            return -1;
        }
        return left;
    }
    //查找指定值最后一次出现的位置
    //找上界
    public static int LastBound(int[] nums, int target) {
        int left=0;
        int right=nums.length-1;
        //跳出条件
        while(left<=right){
            int mid=left+(right-left)/2;
            if(nums[mid]>target){
                right=mid-1;
            }else{
                left=mid+1;
            }
        }
        if(left-1>=nums.length||nums[left-1]!=target){
            return -1;
        }
        return left-1;
    }

    public static void main(String[] args) {
        int[] nums={5,7,7,8,8,10};
        int target=8;
        System.out.println(LastBound(nums,target));
        System.out.println(UpperBound(nums,target));
        System.out.println(FirstBound(nums,target));
        System.out.println(LastBound(nums,target));
    }
}