两数之和

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 (nums[i] + nums[j] == target) {
                return new int[]{i, j};
            }
        }
    }
    return new int[0];
}

public int[] twoSum(int[] nums, int target) {
    Map<Integer, Integer> hashtable = new HashMap<Integer, Integer>();
    for (int i = 0; i < nums.length; ++i) {
        if (hashtable.containsKey(target - nums[i])) {
            return new int[]{hashtable.get(target - nums[i]), i};
        }
        hashtable.put(nums[i], i);
    }
    return new int[0];
}