二分查找、upper_bound、lower_bound
整理及总结二分查找的判断和边界细节
修改版
package com.leej.binarysearch;
import java.util.Arrays;
/**
* @author jerry
* @create 17/10/7 12:21
*/
public class BinarySearch {
public static int BinarySearch(int[] nums, int key) {
int start = 0, end = nums.length - 1;
int mid;
while(start <= end) {
mid = (start + end) >> 1;
if (nums[mid] == key) return mid;
else if (nums[mid] > key)
end = mid -1;
else
start = mid + 1;
}
return -(start + 1);
}
public static int LowerBound(int[] nums, int key) {
int first = 0, last = nums.length;
int mid;
while(first < last) {
mid = (first + last) >> 1;
if (nums[mid] < key) {
first = mid + 1;
} else {
last = mid;
}
}
return first;
}
public static int UpperBound(int[] nums, int key) {
int first = 0, last = nums.length;
int mid;
while(first < last) {
mid = (first + last) >> 1;
if (nums[mid] <= key) {
first = mid + 1;
} else {
last = mid;
}
}
return first;
}
public static void showArrays(int[] nums) {
for(int num : nums) System.out.print(num + " ");
System.out.println();
}
public static void main(String[] args) {
int[] nums = {10,20,30,30,20,10,10,20, 10};
Arrays.sort(nums); //10 10 10 20 20 20 30 30
showArrays(nums);
//System.out.println( BinarySearch(nums, 11) );
System.out.println(LowerBound(nums, 21));
System.out.println(UpperBound(nums, 21));
}
}
实现
import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.Arrays;
import java.lang.String;
/**
* @author jerry
* @create
*/
public class test {
public static void showArray(int[] nums) {
if (nums == null || nums.length == 0) return;
for (int nu : nums)
System.out.printf("%d ", nu);
System.out.println();
}
/**
* 循环条件left<=right, 所以left != mid , right != mid;
* 双闭区间[left, right]
**/
public static int binarySearch(int[] nums, int target) {
int left = 0, right = nums.length - 1, mid; //search range [left, right],闭区间
while(left <= right) {
mid = (left + right) >> 1;
if (nums[mid] == target)
return mid;
else if (nums[mid] < target)
left = mid + 1;
else
right = mid - 1;
}
return -1;
}
/**
**STL 版本lower_bound
* 找到第一个大于等于target的数, in range [first, last), 左开右闭区间
**/
public static int lower_bound(int[] nums, int target) {
int first = 0, last = nums.length;
int mid, len, step;
len = last - first;
while(len > 0) {
step = len >> 1;
mid = first + step;
if (nums[mid] < target) { //[mid + 1, last)
first = mid + 1;
len -= step + 1;
} else { //[first, mid)最后可取边界mid
len = step;
}
}
return first;
}
//找到第一个大于等于target
public static int my_lower_bound(int[] nums, int target) {
int first = 0, last = nums.length, mid;
//in ranget [first, last)
while(first < last) {
mid = (first + last) >> 1;
if (nums[mid] < target) { //[mid+1, last)
first = mid + 1;
} else {
last = mid;
}
}
return first;
}
//找到第一个大于target的数
public static int my_upper_bound(int[] nums, int target) {
int first = 0, last = nums.length, mid;
//in range [first, last)
while(first < last) {
mid = (first + last) >> 1;
if (nums[mid] <= target) {
first = mid + 1;
} else {
last = mid;
}
}
return first;
}
public static void main( String args[] ){
int[] nums = {10,20,30,30,20,10,10,20, 10};
Arrays.sort(nums); //10 10 10 20 20 20 30 30
test.showArray(nums);
System.out.println( test.binarySearch(nums, 11) );
System.out.println( test.my_lower_bound(nums, 20) );
System.out.println( test.lower_bound(nums, 20) );
System.out.println( test.my_upper_bound(nums, 20) );
}
}
//#output
// 10 10 10 10 20 20 20 30 30
// -1
// 4
// 4
// 7
