Hashing

public int[] twoSum(int[] nums, int target) {
        HashMap<Integer, Integer> map = new HashMap<>();
        int[] ans = new int[2];

        for(int i=0; i<nums.length; i++){
            int currEle = nums[i];

            int remainingSum = target - currEle;

            if(map.containsKey(remainingSum)){
                ans[0] = map.get(remainingSum);
                ans[1] = i;
                
                return ans;
            }

            map.put(currEle, i);
        }

        return ans;
    }

    public String frequencySort(String s) {
        HashMap<Character, Integer> fmap = new HashMap<>();

        for(int i=0; i<s.length(); i++){
            char ch = s.charAt(i);

            if(fmap.containsKey(ch)){
                int currFre = fmap.get(ch);
                fmap.put(ch, currFre + 1);
            } else {
                fmap.put(ch, 1);
            }
        }

        // getting all the keys from map
        ArrayList<Character> keys = new ArrayList<>(fmap.keySet());

        PriorityQueue<Character> pq = new PriorityQueue<>((Character t, Character o) -> {
            return fmap.get(o) - fmap.get(t); 
            // o - t => bigger object -> higher priority (max Heap)
            // t - o => smaller object -> higher priority (min heap)
        });

        for(int i=0; i<keys.size(); i++){
            pq.add(keys.get(i));
        }

        StringBuilder sb = new StringBuilder();

        while(pq.size()>0){
            char ch = pq.remove();
            int fre = fmap.get(ch);

            while(fre > 0){
                sb.append(ch);
                fre--;
            }
        }

        return sb.toString();        
    }