Two Sum

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

Naive Approach:

 1  public int[] twoSum(int[] numbers, int target) {
 2         //Note this is the naive apporach
 3         //the time complexity is n^2.
 4         int[] result = new int[2];
 5         for (int i = 0; i < numbers.length; i++) {
 6             for (int j = i + 1; j < numbers.length; j++) {
 7                 if (target == numbers[i] + numbers[j]) {
 8                     //Note
 9                     result[0] = i + 1;
10                     result[1] = j + 1;
11                     return result;
12                 }
13             }
14         }
15         return result;
16     }

Using Hash-Map

 1  public int[] twoSum(int[] numbers, int target) {
 2         int[] result = new int[2];
 3         HashMap<Integer,Integer> map = new HashMap<Integer, Integer>();
 4         for(int i= 0;i<numbers.length;i++){
 5             if(map.containsKey(numbers[i])){
 6                 result[0]=map.get(numbers[i]);
 7                 result[1]=i+1;
 8             }
 9             else {
10                 map.put((target-numbers[i]),i+1);
11             }
12         }
13         return result;
14     }

Basic method of HashMap

http://docs.oracle.com/javase/7/docs/api/java/util/HashMap.html

Modifier and Type	Method and Description
`void`	`clear()` Removes all of the mappings from this map.
`Object`	`clone()` Returns a shallow copy of this `HashMap` instance: the keys and values themselves are not cloned.
`boolean`	`containsKey(Object key)` Returns `true` if this map contains a mapping for the specified key.
`boolean`	`containsValue(Object value)` Returns `true` if this map maps one or more keys to the specified value.
`Set<Map.Entry<K,V>>`	`entrySet()` Returns a `Set` view of the mappings contained in this map.
`V`	`get(Object key)` Returns the value to which the specified key is mapped, or `null` if this map contains no mapping for the key.
`boolean`	`isEmpty()` Returns `true` if this map contains no key-value mappings.
`Set<K>`	`keySet()` Returns a `Set` view of the keys contained in this map.
`V`	`put(K key, V value)` Associates the specified value with the specified key in this map.
`void`	`putAll(Map<? extends K,? extends V> m)` Copies all of the mappings from the specified map to this map.
`V`	`remove(Object key)` Removes the mapping for the specified key from this map if present.
`int`	`size()` Returns the number of key-value mappings in this map.
`Collection<V>`	`values()` Returns a `Collection` view of the values contained in this map.