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https://leetcode.com/problems/two-sum-iii-data-structure-design/

HashTable

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class TwoSum {

    HashMap<Integer,Integer> map;
    /** Initialize your data structure here. */
    public TwoSum() {
        map=new HashMap<>();
    }
    
    /** Add the number to an internal data structure.. */
    public void add(int number) {
        if(this.map.containsKey(number)) map.replace(number,map.get(number)+1);
        else map.put(number,1);
    }
    
    /** Find if there exists any pair of numbers which sum is equal to the value. */
    public boolean find(int value) {
        for(Map.Entry<Integer,Integer> entry:map.entrySet()){
            int complement=value-entry.getKey();
            if(complement!=entry.getKey()){
                if(map.containsKey(complement)) return true;
            }else{
                if(map.get(complement)>1) return true;
            }
        }
        return false;
    }
}

/**
 * Your TwoSum object will be instantiated and called as such:
 * TwoSum obj = new TwoSum();
 * obj.add(number);
 * boolean param_2 = obj.find(value);
 */

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