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