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：

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;
}" data-snippet-id="ext.8d1f812753a04115df01f1e8433b920d" data-snippet-saved="false" data-csrftoken="nURvjSzt-O07nifCL6c8YxB04mRDXqTk8bHs" data-codota-status="done">[code]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;
}