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Implement ternary search #1256

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Merged
StepfenShawn merged 1 commit into from
Apr 18, 2020
Merged

Implement ternary search #1256

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57 changes: 57 additions & 0 deletions src/main/java/com/search/TernarySearch.java
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Original file line number Diff line number Diff line change
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package com.search;

/**
* A ternary search algorithm is a technique in computer science for finding the minimum or maximum of a unimodal function
* The algorithm determines either that the minimum or maximum cannot be in the first third of the domain
* or that it cannot be in the last third of the domain, then repeats on the remaining third.
* <p>
* Worst-case performance Θ(log3(N))
* Best-case performance O(1)
* Average performance Θ(log3(N))
* Worst-case space complexity O(1)
*/
public final class TernarySearch {

/**
* @param arr The **Sorted** array in which we will search the element.
* @param value The value that we want to search for.
* @return The index of the element if found.
* Else returns -1.
*/
public static <T extends Comparable<T>> int find(T[] arr, T value) {
return search(arr, value, 0, arr.length - 1);
}

/**
* @param arr The **Sorted** array in which we will search the element.
* @param key The value that we want to search for.
* @param start The starting index from which we will start Searching.
* @param end The ending index till which we will Search.
* @return Returns the index of the Element if found.
* Else returns -1.
*/
private static <T extends Comparable<T>> int search(T[] arr, T key, int start, int end) {
if (start > end) {
return -1;
}

int mid1 = start + (end - start) / 3;
int mid2 = start + 2 * (end - start) / 3;

if (key.compareTo(arr[mid1]) == 0) {
return mid1;
} else if (key.compareTo(arr[mid2]) == 0) {
return mid2;
}
else if (key.compareTo(arr[mid1]) < 0) {
return search(arr, key, start, --mid1);
}
else if (key.compareTo(arr[mid2]) > 0) {
return search(arr, key, ++mid2, end);
}
else {
return search(arr, key, mid1, mid2);
}
}

}
26 changes: 26 additions & 0 deletions src/test/java/com/search/TernarySearchTest.java
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package com.search;

import org.junit.jupiter.api.Assertions;
import org.junit.jupiter.api.Test;

public class TernarySearchTest {

@Test
void testTernarySearch() {
Integer[] arr1 = {1, 2, 3, 5, 8, 13, 21, 34, 55};
Assertions.assertEquals(2, TernarySearch.find(arr1, 3), "Incorrect index");
Assertions.assertEquals(0, TernarySearch.find(arr1, 1), "Incorrect index");
Assertions.assertEquals(8, TernarySearch.find(arr1, 55), "Incorrect index");
Assertions.assertEquals(-1, TernarySearch.find(arr1, -2), "Incorrect index");
Assertions.assertEquals(-1, TernarySearch.find(arr1, 4), "Incorrect index");

String[] arr2 = {"A", "B", "C", "D"};
Assertions.assertEquals(2, TernarySearch.find(arr2, "C"), "Incorrect index");
Assertions.assertEquals(1, TernarySearch.find(arr2, "B"), "Incorrect index");
Assertions.assertEquals(-1, TernarySearch.find(arr2, "F"), "Incorrect index");

String[] arr3 = {};
Assertions.assertEquals(-1, TernarySearch.find(arr3, ""), "Incorrect index");
}

}