99 lines
3.3 KiB
Java
99 lines
3.3 KiB
Java
package Searches;
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import java.util.Arrays;
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import java.util.Random;
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import java.util.stream.Stream;
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import static java.lang.String.format;
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/**
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* A ternary search algorithm is a technique in computer science for finding the minimum or maximum of a unimodal function
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* The algorithm determines either that the minimum or maximum cannot be in the first third of the domain
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* or that it cannot be in the last third of the domain, then repeats on the remaining third.
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* <p>
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* Worst-case performance Θ(log3(N))
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* Best-case performance O(1)
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* Average performance Θ(log3(N))
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* Worst-case space complexity O(1)
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*
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* @author Podshivalov Nikita (https://github.com/nikitap492)
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* @see SearchAlgorithm
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* @see IterativeBinarySearch
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*/
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public class TernarySearch implements SearchAlgorithm {
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/**
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* @param arr The **Sorted** array in which we will search the element.
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* @param value The value that we want to search for.
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* @return The index of the element if found.
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* Else returns -1.
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*/
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@Override
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public <T extends Comparable<T>> int find(T[] arr, T value) {
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return ternarySearch(arr, value, 0, arr.length - 1);
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}
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/**
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* @param arr The **Sorted** array in which we will search the element.
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* @param key The value that we want to search for.
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* @param start The starting index from which we will start Searching.
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* @param end The ending index till which we will Search.
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* @return Returns the index of the Element if found.
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* Else returns -1.
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*/
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private <T extends Comparable<T>> int ternarySearch(T[] arr, T key, int start, int end) {
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if (start > end) {
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return -1;
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}
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/* First boundary: add 1/3 of length to start */
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int mid1 = start + (end - start) / 3;
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/* Second boundary: add 2/3 of length to start */
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int mid2 = start + 2 * (end - start) / 3;
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if (key.compareTo(arr[mid1]) == 0) {
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return mid1;
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} else if (key.compareTo(arr[mid2]) == 0) {
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return mid2;
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}
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/* Search the first (1/3) rd part of the array.*/
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else if (key.compareTo(arr[mid1]) < 0) {
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return ternarySearch(arr, key, start, --mid1);
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}
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/* Search 3rd (1/3)rd part of the array */
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else if (key.compareTo(arr[mid2]) > 0) {
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return ternarySearch(arr, key, ++mid2, end);
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}
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/* Search middle (1/3)rd part of the array */
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else {
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return ternarySearch(arr, key, mid1, mid2);
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}
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}
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public static void main(String[] args) {
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//just generate data
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Random r = new Random();
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int size = 100;
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int maxElement = 100000;
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Integer[] integers = Stream.generate(() -> r.nextInt(maxElement)).limit(size).sorted().toArray(Integer[]::new);
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//the element that should be found
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Integer shouldBeFound = integers[r.nextInt(size - 1)];
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TernarySearch search = new TernarySearch();
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int atIndex = search.find(integers, shouldBeFound);
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System.out.println(format("Should be found: %d. Found %d at index %d. An array length %d"
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, shouldBeFound, integers[atIndex], atIndex, size));
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int toCheck = Arrays.binarySearch(integers, shouldBeFound);
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System.out.println(format("Found by system method at an index: %d. Is equal: %b", toCheck, toCheck == atIndex));
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}
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} |