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].

初始代码

利用了两个指针。

class Solution {
    public int[] twoSum(int[] nums, int target) {
    
        ArrayList<Integer> list = new ArrayList<Integer>();
        ArrayList<Integer> unSortedlist = new ArrayList<Integer>();
        for(int i:nums){
            list.add(i);
            unSortedlist.add(i);
        }
        Collections.sort(list);
   
        int first = 0;
        int last = list.size()-1;
        
        while(last>first){
            if(list.get(first)+list.get(last)<target){
                first++;
            }else if(list.get(first)+list.get(last)>target){
                last--;
            }else{
                break;
            }
        }
        
        ArrayList<Integer> valueTwoList = new ArrayList<Integer>();
        for(int i = 0;i<nums.length;i++){
            if(nums[i] ==list.get(first)||nums[i] ==list.get(last)){
                valueTwoList.add(i);
                break;
            }
        }
        for(int i = nums.length-1;i>=0;i--){
            if(nums[i] ==list.get(first)||nums[i] ==list.get(last)){
                valueTwoList.add(i);
                break;
            }
        }
        
        Collections.sort(valueTwoList);
        int result[] = {valueTwoList.get(0),valueTwoList.get(1)};
        return result;
       
        
        
    }
}

大神代码 利用了containskey ，相当厉害了。

public int[] twoSum(int[] nums, int target) {
    
        int[] result = new int[2];
        Map<Integer, Integer> map = new HashMap<Integer, Integer>();
        for (int i = 0; i < nums.length; i++) {
            if (map.containsKey(target - nums[i])) {
                result[1] = i;
                result[0] = map.get(target - nums[i]);
                return result;
        }
        map.put(nums[i], i );
        }
        return result;
          
    }