二分法變種

package com

public class Test {
    public static void main(String[] args) {
        int[] nums = {4,7,7,9,12,13};
        int index = searchLastSmaller(nums,1);
        System.out.println(index);
    }
    
    /*
     * 第一個與key相等的元素
     */
    public static int searchFirstEqual(int[] arr, int key){
        int left = 0, right = arr.length-1;
        while(left<=right){
            int mid = (left+right)/2;
            if(arr[mid]>=key)right=mid-1;
            else if(arr[mid]<key) left = mid+1;
        }
        if(left<arr.length&&arr[left]==key) return left;
        return -1;
    }
    
    /*
     * 找出最後一個與key相等的元素
     */
    public static int searchLastEqual(int[] arr, int key){
        int left = 0, right = arr.length-1;
        while(left<=right) {
            int mid = (left+right)/2;
            if(arr[mid] > key) right = mid - 1;
            else if(arr[mid] <= key) left = mid + 1; 
        }
        if( right>=0 && arr[right] == key) return right;
        return -1;
    }
    
    /*
     * 找出第一個大於等於key的元素
     * 
     */
    
    public static int searchFirstEqualOrLarger(int arr[],int key){
        int left=0, right=arr.length-1;
        while(left<=right) {
            int mid = (left+right)/2;
            if(arr[mid] >= key) right = mid-1;
            else if (arr[mid] < key) left = mid+1;
        }
        if(left>=arr.length)return -1;
        return left;
    }
    
    /*
     * 查找第一個大於key的元素
     */
    public static int searchFirstLarger(int[] arr, int key){
        int left=0, right=arr.length-1;
        while(left<=right) {
            int mid = (left+right)/2;
            if(arr[mid] > key) right = mid-1;
            else if (arr[mid] <= key) left = mid+1;
        }
        if(left>=arr.length)return -1;
        return left;
    }
    
    /*
     * 查找最後一個小於或等於Key的元素
     */
    public static int searchLastEqualOrSmaller(int[] arr, int key){
        int left=0, right=arr.length-1;
        while(left<=right) {
            int m = (left+right)/2;
            if(arr[m] > key) right = m-1;
            else if (arr[m] <= key) left = m+1;
        }
        //若right=-1，表明不存在
        return right;
    }
    
    /*
     * 查找最後一個小於key的元素
     */
    public static int searchLastSmaller(int[] arr, int key){
        int left=0, right=arr.length-1;
        while(left<=right) {
            int mid = (left+right)/2;
            if(arr[mid] >= key) right = mid-1;
            else if (arr[mid] < key) left = mid+1;
        }
        //若right=-1，表明不存在
        return right;
    }

}