LeetCode-Two Sum III - Data structure design

Posted 2020-08-08 LiBlog

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

 

 Analysis:

Use O(1) add and O(n) find.

 

Solution:

public class TwoSum {
    HashMap<Integer,Integer> numCounts = new HashMap<Integer,Integer>();
    List<Integer> nums = new ArrayList<Integer>();

    // Add the number to an internal data structure.
    public void add(int number) {
        if (numCounts.containsKey(number)){
            numCounts.put(number,numCounts.get(number)+1);
        } else {
            nums.add(number);
            numCounts.put(number,1);
        }
    }

    // Find if there exists any pair of numbers which sum is equal to the value.
    public boolean find(int value) {
        for (int num : nums){
            int otherNum = value - num;
            boolean validPair = (otherNum==num) ? (numCounts.get(num)>=2) : numCounts.containsKey(otherNum);
            if (validPair){
                return true;
            }
        }
        return false;
    }
}


// Your TwoSum object will be instantiated and called as such:
// TwoSum twoSum = new TwoSum();
// twoSum.add(number);
// twoSum.find(value);