建立測試案例

public class LeetCode1TwoSumTest {

    final LeetCode1TwoSum twoSum = new LeetCode1TwoSum();

    /**
     * Example:
     * Given nums = [2, 7, 11, 15], target = 9,
     * Because nums[0] + nums[1] = 2 + 7 = 9,
     * return [0, 1].
     */
    @Test
    public void test1() {
        final int[] nums = new int[]{2, 7, 11, 15};
        final int target = 9;

        int[] act = twoSum.twoSum(nums, target);

        Assert.assertTrue(IntStream.of(act).anyMatch(x -> x == 0));
        Assert.assertTrue(IntStream.of(act).anyMatch(x -> x == 1));
    }

    @Test
    public void test2() {
        final int[] nums = new int[]{3, 3};
        final int target = 6;

        int[] act = twoSum.twoSum(nums, target);

        Assert.assertTrue(IntStream.of(act).anyMatch(x -> x == 0));
        Assert.assertTrue(IntStream.of(act).anyMatch(x -> x == 1));
    }

    @Test
    public void test3() {
        final int[] nums = new int[]{3, 2, 4};
        final int target = 6;

        int[] act = twoSum.twoSum(nums, target);

        Assert.assertTrue(IntStream.of(act).anyMatch(x -> x == 1));
        Assert.assertTrue(IntStream.of(act).anyMatch(x -> x == 2));
    }
}

Solution

public class LeetCode1TwoSum {

    public int[] twoSum(final int[] nums, final int target) {
        // 用來存放 target - n , HashMap
        HashMap map = new HashMap<>();
        int[] result = new int[2];

        for (int i = 0; i < nums.length; i++) {
            // 如果 map 有(target - n) 的值, 依序將 index 存入 result, 回傳 result
            if (map.containsKey(nums[i])) {
                int index = map.get(nums[i]);
                result[0] = index;
                result[1] = i;
                return result;
            } else {
                // 計算目前 index , 相差(target - n) 才會等於 target
                map.put(target - nums[i], i);
            }
        }
        return result;
    }
}