二分查找
二分查找时间复杂度： O ( l o g n ) O(logn) O(logn)
直接上代码
Java:
public class BinarySearch {
// has to return boxed integer in order to comfort to interface defined in the book
private static Integer binarySearch(int[] list, int item) {
int low = 0;
int high = list.length - 1;
while (low <= high) {
int mid = (low + high) / 2;
int guess = list[mid];
if (guess == item) {
return mid;
}
if (guess > item) {
high = mid - 1;
} else {
low = mid + 1;
}
}
return null;
}
public static void main(String[] args) {
int[] myList = {
1, 3, 5, 7, 9};
System.out.println(binarySearch(myList, 3)); // 1
System.out.println(binarySearch(myList, -1)); // null
}
}
C：
#include <stdio.h>
int binarySearch(int[], int, int);
int main()
{
int myList[] = {
1, 3, 5, 7, 9};
int len = sizeof(myList) / sizeof(myList[0]);
printf("%d\n", binarySearch(myList, 3, len)); // 1
printf("%d\n", binarySearch(myList, -1, len)); //-1
return 0;
}
int binarySearch(int list[], int item, int len)
{
int low = 0;
int high = len;
while (low <= high)
{
int mid = (low + high)/2;
int guess = list[mid];
if (guess == item)
{
return mid;
}
else if (guess > item)
{
high = mid - 1;
}
else
{
low = mid + 1;
}
}
return -1; //number not found
}
C++：
#include "iostream"
using namespace std;
void binarySearch(int data_array[], int element, int len)
{
int low = 0;
int high = len;
while (low <= high)
{
int mid = (low + high)/2;
int guess = data_array[mid];
if (guess == element)
{
cout<<"Element found at "<<mid<<" th index"<<endl ;
return ;
}
else if (guess > element)
{
high = mid - 1;
}
else
{
low = mid + 1;
}
}
cout<<"Element Not Found"<<endl ;
return ; //number not found
}
int main()
{
int data_array[] = {
2,10,23,44,100,121};
int length = sizeof(data_array) / sizeof(int);
binarySearch(data_array, 3, length) ; // not found case
binarySearch(data_array, 2, length) ; // found at corner case
binarySearch(data_array, 44, length) ; //found at middle case
return 0;
}
Python：
class BinarySearch():
def search_iterative(self, list, item):
# low and high keep track of which part of the list you'll search in.
low = 0
high = len(list) - 1
# While you haven't narrowed it down to one element ...
while low <= high:
# ... check the middle element
mid = (low + high) // 2
guess = list[mid]
# Found the item.
if guess == item:
return mid
# The guess was too high.
if guess > item:
high = mid - 1
# The guess was too low.
else:
low = mid + 1
# Item doesn't exist
return None
def search_recursive(self, list, low, high, item):
# Check base case
if high >= low:
mid = (high + low) // 2
guess = list[mid]
# If element is present at the middle itself
if guess == item:
return mid
# If element is smaller than mid, then it can only
# be present in left subarray
elif guess > item:
return self.search_recursive(list, low, mid - 1, item)
# Else the element can only be present in right subarray
else:
return self.search_recursive(list, mid + 1, high, item)
else:
# Element is not present in the array
return None
if __name__ == "__main__":
# We must initialize the class to use the methods of this class
bs = BinarySearch()
my_list = [1, 3, 5, 7, 9]
print(bs.search_iterative(my_list, 3)) # => 1
# 'None' means nil in Python. We use to indicate that the item wasn't found.
print(bs.search_iterative(my_list, -1)) # => None