【leetcode 两数之和】

import java.util.HashMap;

//leetcode submit region begin(Prohibit modification and deletion)
class Solution {
    public int[] twoSum(int[] nums, int target) {

        // 暴力枚举 O(n^2)
        //for (int i = 0; i < nums.length; i++) {
        //    for (int j = i + 1; j < nums.length; j++) {
        //        if (nums[i] + nums[j] == target)
        //            return new int[]{i, j};
        //    }
        //}
        //return new int[0];

        // Hash匹配 O(n)
        HashMap<Integer, Integer> hashMap = new HashMap<>();
        for (int i = 0; i < nums.length; i++) {
            if (hashMap.containsKey(target - nums[i])) {
                return new int[]{hashMap.get(target - nums[i]), i};
            }
            hashMap.put(nums[i], i);
        }
        return new int[0];

    }
}