leetcode1. Two Sum

1. Two Sum

Given an array of integers, return indices of the two numbers such that they add up to a specific target.

You may assume that each input would have exactly one solution, and you may not use the same element twice.

Example:Given nums = [2, 7, 11, 15], target = 9,Because nums[0] + nums[1] = 2 + 7 = 9,return [0, 1].

解法一

暴力

class Solution {    /**     * @time O(n^2)     * @space O(1)     */    public int[] twoSum(int[] nums, int target) {        if (nums == null || nums.length == 0) {            return null;        }        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 null;    }}

解法二

hashtable

public class Solution {    /*     * @time O(n)     * @space O(n)     */    public int[] twoSum(int[] nums, int target) {        Map<Integer, Integer> hashMap = new HashMap<>();        for (int i = 0; i < nums.length; i++) {            if (hashMap.containsKey(nums[i])) {                int[] result = {hashMap.get(nums[i]), i};                return result;            }             hashMap.put(target - nums[i], i);        }        int[] result = {};        return result;    }}