1. 两数之和

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

        }
        return null;
    }
}

class Solution {
    public int[] twoSum(int[] nums, int target) {

        HashMap<Integer, Integer> map = new HashMap<>();
        for (int i = 0; i < nums.length; i++) {

            if (map.get(nums[i]) != null) {
                return new int[]{map.get(nums[i]), i};
            }
            map.put(target - nums[i], i);
        }

        return new int[]{-1, -1};
    }
}