14-1. Two Sum

/*class Solution {
    public int[] twoSum(int[] nums, int target) {
        int sum=0;
        for (int i = 0; i < nums.length; i++){
            for (int j = nums.length-1; j > 0; j--){
                sum=nums[i]+nums[j];
                if(sum==target){
                    return new int[] {i,j};
                    }
            }
        }
        return null;
    }
}*/

//2nd SOLUTION-OPTIMIZED

class Solution {
    public int[] twoSum(int[] nums, int target) {
        int pair[]=new int[2];
        HashMap<Integer,Integer> map=new HashMap<>();
        for(int i=0;i<nums.length;i++){
            int result=target-nums[i];
            if(map.get(result)!=null){
                pair[0]=map.get(result);
                pair[1]=i;
            }
            else{
                map.put(nums[i],i);
            }
        }
        return pair;
    }
}

// In the map, keys and values are paired as key:value.and we can call only keys.
 https://leetcode.com/problems/two-sum/