个人记录-LeetCode 1.Two Sum

问题：
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.

代码示例：
1、暴力解法

public class Solution {
    public int[] twoSum(int[] nums, int target) {
        if(nums == null || nums.length < 2) {
            return null;
        }

        for(int i = 0; i < nums.length - 1; ++i) {
            for (int j = i + 1; j < nums.length; ++j) {
                if (nums[i] + nums[j] == target) {
                    return new int[] {i, j};
                }
            }
        }

        return null;
    }
}

2、利用hashMap来快速查找

public int[] twoSum(int[] nums, int target) {
    Map<Integer, Integer> map = new HashMap<>();
    for (int i = 0; i < nums.length; i++) {
        //求出需要的匹配数字
        int complement = target - nums[i];
        //判断匹配数字是否已经加入到map中
        //更快，而且可以避免加入重复数字时HashMap的替换
        if (map.containsKey(complement)) {
            return new int[] { map.get(complement), i };
        }
        map.put(nums[i], i);
    }
    throw new IllegalArgumentException("No two sum solution");
}