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].

class Solution {
    public int[] twoSum(int[] nums, int target) {
        int[] result = new int[2];

        HashMap<Integer,Integer> hashmap = new HashMap<>();

        //List arrayList = Arrays.asList(nums);

        HashMap<Integer,Integer> index = new HashMap<>();
        for(int i=0;i<nums.length;i++){
            if(index.get(nums[i])==null)
                index.put(nums[i],i);
            else if(2*nums[i]==target){
                result[0]=i;
                result[1]=index.get(nums[i]);
                return result;
            }
        }

        for(int i=0;i<nums.length;i++){
            //if(hashmap.get(target-nums[i])==null)
            hashmap.put(target-nums[i],nums[i]);
        }

        //Set set = hashmap.keySet();
        for(int i=0;i<nums.length;i++){
            if(hashmap.get(nums[i])!=null){
                if(index.get(nums[i])!=index.get(target-nums[i])){
                    result[0]=index.get(nums[i]);
                    result[1]=index.get(target-nums[i]);
                }
                
            }
        }
        return result;
    }
}

兩個特殊的地方：

1.

target=6,int[]{3,3}

2.

target=28,int[]{1,27,14}

剛好有目標值的一半