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].

//Time: O(n), Space: O(1)    
public int[] twoSum(int[] nums, int target) {
        if (nums == null || nums.length == 0) {
            return null;
        }
        
        Map<Integer, Integer> map = new HashMap<Integer, Integer>();
        int[] result = new int[2];
        
        for (int i = 0; i < nums.length; i++) {
            if (map.containsKey(target - nums[i])) {
                result[0] = map.get(target - nums[i]);
                result[1] = i;
                break;//注意这里不要忘记跳出
            }
            
            map.put(nums[i], i);//不要忘记随时更新map
        }
        
        return result;
    }