算法---從一個數組(或者集合中)找出和爲某個值的下標

需求，例如：

Given nums = [2, 7, 11, 15], target = 9,
Because nums[0] + nums[1] = 2 + 7 = 9,
return [0, 1].

    public static void main(String[] args) {
        int[] nums = new int[]{2, 7, 5, 1, 3};
        int[] ints = twoSum(nums, 10);
        System.out.println(ints[0]);
        System.out.println(ints[1]);
    }

    public static int[] twoSum(int[] nums, int target) {
        //因爲你要找到這兩個相加等於目標數,因此我認爲你至少要遍歷一次
        Map hashMap = new HashMap<>();
        for (int i = 0; i < nums.length; i++) {
            int complement = target - nums[i];
            if (hashMap.containsKey(complement)) {
                return new int[]{hashMap.get(complement), i};
            }
            hashMap.put(nums[i], i);
        }
        throw new IllegalArgumentException("No two sum solution");
    }

public static int[] twoSum(int[] nums, int target) {
        for (int i = 0; i < nums.length; i++) {
            for (int j = i + 1; j < nums.length; j++) {
                if (nums[j] == target - nums[i]) {
                    return new int[]{i, j};
                }
            }
        }
        throw new IllegalArgumentException("No two sum solution");
    }