给定一个整数数组和一个目标值，找出数组中和为目标值的两个数。

你可以假设每个输入只对应一种答案，且同样的元素不能被重复利用。

示例:

给定 nums = [2, 7, 11, 15], target = 9

因为 nums[0] + nums[1] = 2 + 7 = 9
所以返回 [0, 1]

1.第一想法

双层for循环

java:

private static int[] twoSumDoubleLoop(int[] nums, int target) {
    for (int i = 0; i<nums.length-1; i++){
        for (int j=i+1; j<nums.length; j++){
            if (nums[i]+nums[j] == target){
                return new int[]{i, j};
            }
        }
    }
    return null;
}

scala:

def twoSumDoubleLoop(nums: Array[Int], target: Int): Array[Int] = {
    for (i <- 0 until nums.length-1; j <- i + 1 until nums.length){
      if (nums(i)+nums(j) == target){
        return Array(i, j)
      }
    }
    throw new Exception("not found@!")
  }

2.第二想法

映射表实现
java:

private static int[] twoSumHashMap(int[] nums, int target) {
    int[] res = new int[2];
    HashMap<Integer, Integer> map = new HashMap<>();
    for (int i = 0; i < nums.length; i++){
        if (map.containsKey(nums[i])){
            res[0] = map.get(nums[i]);
            res[1] = i;
        }
        map.put(target-nums[i], i);
    }
    return res;
}

scala:

def twoSumHashMap(nums:Array[Int], target:Int): Array[Int] = {
    val map = new mutable.HashMap[Int, Int]()
    for (i <- nums.indices){
      if (map.contains(nums(i))){
        return Array(map(nums(i)), i)
      }
//      map(target-nums(i)) = i
      map.+=((target-nums(i),i))
    }
    throw new Exception("not found@!")
  }