q1_两数之和

/**
 * 暴力法 o(n^2)
 */
class Solution {
    public int[] twoSum(int[] nums, int target) {
        int[] result = new int[2];
        for (int i = 0; i < nums.length; i++) {
            for (int j = i + 1; j < nums.length; j++) {
                if (nums[i] + nums[j] == target) {
                    result[0] = i;
                    result[1] = j;
                    return result;
                }
            }
        }
        return null;
    }
}
import java.util.HashMap;
import java.util.Map;
/**
 * 一遍hash o(n)
 */
public 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])) {
                return new int[]{map.get(target - nums[i]), i};
            }
            map.put(nums[i], i);
        }
        return null;
    }
}