1. Two Sum

//O(n) time and O(n) space
class Solution {
    public int[] twoSum(int[] nums, int target) {
        int[] result = new int[2];
        int n = nums.length;
        Map<Integer, Integer> map = new HashMap();
        for(int i=0;i<n;i++){
            if(map.containsKey(target-nums[i])){
                result[0] = map.get(target-nums[i]);
                result[1] = i;
                return result;
            }
            map.put(nums[i], i);
        }
        return result;
    }
}

//O(n^2) time and O(1) space
class Solution {
    public int[] twoSum(int[] nums, int target) {
        int[] result = new int[2];
        int n = nums.length;
        for(int i=0;i<n;i++){
            for(int j=i+1;j<n;j++){
                int sum = nums[i] + nums[j];
                if(sum==target){
                    result[0] = i;
                    result[1] = j;
                }
            }
        }
        return result;
    }
}