『LeetCode』1. 两数之和 Two Sum

『1』暴力法

class Solution {
    // Brute Force
    // Time Complexity: O(n^2)
    // Space Complexity: O(1)
    public int[] twoSum(int[] nums, int target) {
        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) {
                    return new int[] {i, j};
                }
            }
        }
        return new int[0];
    }
}

『2』哈希表法

class Solution {
    // HashMap
    // Time Complexity: O(n)
    // Space Complexity: O(n)
    public int[] twoSum(int[] nums, int target) {
        HashMap<Integer, Integer> map = new HashMap<>();
        for (int i = 0; i < nums.length; i++) {
            int diff = target - nums[i];
            if (map.containsKey(diff)) {
                return new int[] {i, map.get(diff)};
            }
            map.put(nums[i], i);
        }
        return new int[0];
    }
}