TwoSum1

问题描述

Given an array of integers, find two numbers such that they add up to a specific target number.The function twoSum should return indices of the two numbers such that they add up to the target, where index1 must be less than index2. Please note that your returned answers (both index1 and index2) are not zero-based.
You may assume that each input would have exactly one solution.
Input: numbers={2, 7, 11, 15}, target=9
Output: index1=1, index2=2

算法描述

第一种解法

首先我们用一个List<Node>存储Node，其中Node的成员变量包括val(数组中的值)和idx(数组值对应的索引+1)，然后对这个List按Node.val进行排序，排序之后，我们用两个指针索引left和right分别从左和右来判断它们的和是否等于target。

• 如果等于，直接返回相应的索引（从小到大）
• 如果大于，right–
• 如果小于，left++

其代码实现如下：

public int[] twoSum(int[] nums, int target){
        if(nums == null || nums.length == 0)
            return null;
        List<Node> list = new ArrayList<>();
        for(int i = 0; i < nums.length; i++){
            list.add(new Node(i+1, nums[i]));
        }
        list.sort(new Comparator<Node>() {
            @Override
            public int compare(Node o1, Node o2) {
                return o1.val - o2.val;
            }
        });
        int left = 0;
        int right = nums.length - 1;
        while(left < right){
            Node node1 = list.get(left);
            Node node2 = list.get(right);
            int sum = node1.val + node2.val;
            if(target == sum){
            return new int[]{Math.min(node1.idx, node2.idx),
                             Math.max(node1.idx, node2.idx)};
            }else if(target > sum){
               left++;
            }else {
               right--;
            }
        }
        return null;
    }
    public class Node{
        int idx;
        int val;
        public Node(int idx, int val){
            this.idx = idx;
            this.val = val;
        }
    } 

第二种解法

我们采用Hash结构来存储数组中的元素val和对应的索引号idx+1。

• 如果target - nums[i]不在Map中，则直接把nums[i]和i+1放进Map中
• 如果target - nums[i]在Map中，则直接返回target - nums[i]对应的idx 和 i+1

相应的代码实现：

public int[] twoSumByHash(int[] nums, int target){
        if(nums == null)
            return null;

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