Two Sum III – Data structure design (Java)

Category: Algorithms >> Interview March 29, 2014

Design and implement a TwoSum class. It should support the following operations: add and find.

add - Add the number to an internal data structure.
find - Find if there exists any pair of numbers which sum is equal to the value.

For example,

add(1); 
add(3); 
add(5);
find(4) -> true
find(7) -> false

Java Solution

Since the desired class need add and get operations, HashMap is a good option for this purpose.

public class TwoSum {
	private HashMap<Integer, Integer> elements = new HashMap<Integer, Integer>();
 
	public void add(int number) {
		if (elements.containsKey(number)) {
			elements.put(number, elements.get(number) + 1);
		} else {
			elements.put(number, 1);
		}
	}
 
	public boolean find(int value) {
		for (Integer i : elements.keySet()) {
			int target = value - i;
			if (elements.containsKey(target)) {
				if (i == target && elements.get(target) < 2) {
					continue;
				}
				return true;
			}
		}
		return false;
	}
}

If you want someone to read your code, please put the code inside <pre><code> and </code></pre> tags. For example:

<pre><code> 
String foo = "bar";
</code></pre>

alexwest11

put all numbers in hash.

for each number in hash, find in hash [value- number], if yes. such pair exist!

======
no need any freq table. !! it is misleading. we need put counter if in array [1 2 4 3 4 ] we are looking for 8
, so need to know exist 2 of them.

so instead of incr, if exist already, put any number not 1. thus we know it is more than 1 of 4!
alexwest11

elements.put(number, elements.get(number) + 1);
======

this line is confusing. no need one! just put 2 or anything not 1 . that’s all

it is done to find in [1 2 4 3 4 ] value 8.
Michael Brooks

Python3

class TwoSum:
def __init__(self):
self.values = set()

def add_me(self, num):
self.values.add(num)

def find_me(self, num):
myNum = num
seen = set()
for val in self.values:
temp = num – val
if temp in self.values:
return True
return False
Cat Racket

A JavaScript Solution with Youtube Video Explains:
Chinese: https://www.youtube.com/watch?v=eM7GZHgclVU
Facebook: https://www.facebook.com/groups/2094071194216385/
Ahmed Hamdy

He used hashtable as a frequency table, add will work in O(1) and find will work in O(n)
add: will check if the number already in the table, if so, increment the count by 1, else, add the number to the table.
find: will loop over the table and for each element, he will compute the (element – value), like if the value is 9 and the current element in the loop is 5, then the target should be 9 – 5 = 4, if 4 found in the table, then return true, else, check the next element and so on. If none found, return false.
Tao Cheng

can anyone explain the solution please
Soran Ebrahim

Unfortunately TLE, actually I tried this before and out of my luck. So I’m here to see if there’s a better solution, but still unable to find one:(
kingdaa

Elegant solution.

Two Sum III – Data structure design (Java)

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