package TwoSum;

import java.util.HashMap;

import java.util.HashSet;

import java.util.Map;

import java.util.Set;

public class TwoSumDemo {

public static void main(String[] args) {

int[] table = {1, 2, 10, 15, 7, 3, 20, 100, 25, 96, 4, 5, 6, 8, 9, 11};

int target = 13;

int[] result = findTwoSumMap(table, target);

if (result != null)

System.out.println(target + " = " + table[result[0]] + " + " + table[result[1]]);

}

// solution #1: using brute force (complexity: O(n²))

public static int[] findTwoSumBruteForce(int[] array, int n) {

for (int i = 0; i < array.length; i++) {

for (int j = 0; j < array.length; j++) {

if (i == j)

continue;

if (array[i] + array[j] == n) {

return new int[]{i, j};

}

}

}

return null;

}

// solution #2: using map data structure (complexity: O(n))

public static int[] findTwoSumMap(int[] array, int target) {

Map<Integer, Integer> visitedNumbers = new HashMap<>();

for (int i = 0; i < array.length; i++) {

int neededNum = target - array[i];

if(visitedNumbers.containsKey(neededNum)) {

return new int[]{i, visitedNumbers.get(neededNum)};

}

visitedNumbers.put(array[i], i);

}

return null;

}

// solution #3: using set data structure (complexity: O(n))

public static int[] findTwoSumSet(int[] array, int n) {

Set<Integer> numbers = new HashSet<>();

for (int i = 0; i < array.length; i++) {

numbers.add(array[i]);

}

int firstIndice = -1, target = -1;

boolean isFound = false;

for (int i = 0; i < array.length; i++) {

target = n - array[i];

if (target == array[i] || target < 0)

continue;

if (numbers.contains(target)) {

isFound = true;

firstIndice = i;

break;

}

}

if (isFound) {

for (int i = 0; i < array.length; i++) {

if (target == array[i])

return new int[]{firstIndice, i};

}

}

return null;

}

}