解题思路：

解法一：暴力

class Solution {
    public int[] twoSum(int[] nums, int target) {
        int n = nums.length;
        for (int i = 0; i < n; i++)
            for (int j = i + 1; j < n; j++) {
                if (target == nums[i] + nums[j])
                    return new int[] { i, j };
            }
        return new int[0];
    }
}

解法二：哈希表

class Solution {
    public int[] twoSum(int[] nums, int target) {
        Map<Integer, Integer> map = new HashMap<>();
        for (int i = 0; i < nums.length; ++i) {
            if (map.containsKey(target - nums[i])) {
                //保证返回的下标由小到大
                return new int[]{map.get(target - nums[i]), i};
            }
            map.put(nums[i], i);
        }
        return new int[0];
    }
}