Leetcode练习题Two Sum

1 Two Sum:

Question
Solution
知识点总结
常见方法
HashMap由value获得key

Question：

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].

Solution:

class Solution {

    public int[] twoSum(int[] nums, int target) {

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

        for(int i=0;i<nums.length;i++)

        {

            hashmap.put(nums[i],i);

        }

        int v1=0,v2=0;

        for(int m=0;m<nums.length;m++)

        {

            int complement = target-nums[m];

            if (hashmap.containsKey(complement)&&( hashmap.get(complement)!=m))

            {

                return new int[] {m,hashmap.get(target-nums[m])};

            }

        }

        throw new IllegalArgumentException("No two sum solution");

    }

}

知识点总结：

ClassMap<K,V>

K - the type of keys maintained by this map

V - the type of mapped values

常见方法：

clear:

Removes all of the mappings from this map.

containsKey:

Returns true if this map contains a mapping for the specified key.

containsValue:

Returns true if this map maps one or more keys to the specified value.

get:

Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.

put:

Associates the specified value with the specified key in this map.

size:

Returns the number of key-value mappings in this map.

replace:

Replaces the entry for the specified key only if it is currently mapped to some value.

isEmpty:

Returns true if this map contains no key-value mappings.

remove:

Removes the mapping for the specified key from this map if present.

keySet:

Returns a Set view of the keys contained in this map.

HashMap由value获得key:

由于hashmap中key值唯一，而value值不唯一；所以一般都是通过get函数实现,获得value值；而如果想通过value获得key这需要自己写，可通过如下操作；

查找一个key值：

public class HashMap

{

	public static String getKey(HashMap<Integer,Integer> hashmap,int v alue)

	{

		int findValue = 0;

		//迭代循环

		for(Integer getKey:hashmap.keySet())

		{

			if(hashmap.get(getKey).equals(findValue))

			{

				findValue = getKey

			}

		}

		return findValue;

	}

}

查找一个key集合

public static List<Integer> getKeyList(HashMap<Integer,Ingeger> hashmap, int value)

{

	List<Integer> list = new ArrayList();

	for(Integer getKey:hashmap.keySet())

	{

		if(hashmap.get(getKey).equals(value))

		{

			list.add(getKey);

		}

	}

	return list;

}

References:

HashMap API

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