Two_Sum.java

//LeetCode 1. Two Sum
//Question Link - https://leetcode.com/problems/two-sum/

classSolution {
publicint[] twoSum(int[] nums, inttarget) {
intresult[] = newint[2];

//to store the numbers which have already been encountered
//key = number in array, value = index of the number
Map<Integer, Integer> seen = newHashMap<>();

for(inti = 0 ; i < nums.length ; i++){
//the number nums[i] is alreday in the array
//check if (target - nums[i]) is also in the array or not 
intcomplement = target - nums[i];

//we check if we have already encountered complement or not.
if(seen.containsKey(complement)){
returnnewint[]{seen.get(complement), i};
 }

//if complement does not exsist, we must store the current number in seen
//The current number could be the complement of some number to be visited further in the array
seen.put(nums[i], i);
 }

returnresult;
 }
}