Leetcode—两数之和1

给定一个整数数组nums和一个正阿虎目标值target,请你在该数组中找出和为目标值的那两个整数，并返回他们的数组下标

你可以假设每种输入只会对应一个答案，但是，数组中同一个元素不能使用两遍。

示例：

nums = [2,7,11,15]，target = 9；

输出：[0,1]

解法一：暴力法

class Solution {
    // Brute Force
    // N is the size of nums
    // Time Complexity: O(N^2)
    // Space COmplexity: O(1)
    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++) {
                int sum = nums[i] + nums[j];
                if (sum == target) {
                    result[0] = i;
                    result[1] = j;
                    return result;
                }
            }
        }
        return result;
    }
}

解法二：哈希表法

class Solution {
    // HashMap
    // N is the size of nums
    // Time Complexity: O(N)
    // Space COmplexity: O(N)
    public int[] twoSum(int[] nums, int target) {
        int[] result = new int[2];
        HashMap<Integer, Integer> map = new HashMap<>();
        for (int i = 0; i < nums.length; i++) {
            map.put(nums[i], i);
        }
        for (int j = 0; j < nums.length; j++) {
            int diff = target - nums[j];
            if (map.containsKey(diff) && map.get(diff) != j) {
                result[0] = j;
                result[1] = map.get(diff);
                return result;
            }
        }
        return result;
    }
}