1. Two Sum

 JAVA

• class Solution {
      public int[] twoSum(int[] nums, int target) {
          Map<Integer, Integer> m = new HashMap<>();
          for (int i = 0;; ++i) {
              int x = nums[i];
              int y = target - x;
              if (m.containsKey(y)) {
                  return new int[] {m.get(y), i};
              }
              m.put(x, i);
          }
      }
  }

C++

• class Solution {
  public:
      vector<int> twoSum(vector<int>& nums, int target) {
          unordered_map<int, int> m;
          for (int i = 0;; ++i) {
              int x = nums[i];
              int y = target - x;
              if (m.count(y)) {
                  return {m[y], i};
              }
              m[x] = i;
          }
      }
  };

JAVASCRIPT

var twoSum = function (nums, target) {
    const m = new Map();
    for (let i = 0; ; ++i) {
        const x = nums[i];
        const y = target - x;
        if (m.has(y)) {
            return [m.get(y), i];
        }
        m.set(x, i);
    }
};