two sum 3道题

I 非排好序，最快O（n）， 用hashmap解决

 

public class Solution {
    public int[] twoSum(int[] numbers, int target) {
        int[] res  =  new int[2];
        HashMap<Integer, Integer> hm = new HashMap<Integer, Integer>();
        if(numbers==null || numbers.length<2) return res;
        
        for(int i=0; i<numbers.length; i++){
            if(!hm.containsKey(target-numbers[i])){
                hm.put(numbers[i], i);
            }else{
                res[0] = hm.get(target-numbers[i])+1;
                res[1] = i+1;
                break;
            }
        }
       return res;
    }
}

 

 

 

II 排好序数组，2 pointers解决

 

public class Solution {
    public int[] twoSum(int[] numbers, int target) {
        int[] res  =  new int[2];
        int sum = 0, low =0, high = numbers.length-1;
        if(numbers==null || numbers.length<2) return res;
        
       while(low<high){
           sum = numbers[low]+numbers[high];
           if(sum==target){
               res[0] = low+1;
               res[1] = high+1;
               break;
           }else if(sum>target){
               high--;
           }else
                low++;
       }
       return res;
    }
}

 

III

 

 

Two Sum III - Data structure design (LeetCode HashMap)

 

Question:  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

 

 

注意其中如果target ==2 elements in set怎么做判断，见 ref ：http://shanjiaxin.blogspot.com/2015/01/two-sum-iii-data-structure-design.html

 

copy其code：

 

import java.util.HashMap;
import java.util.Map;

// case: add(0) -> find(0) -> should be false
public class TwoSumIII {
    Map<Integer, Integer> dict = new HashMap<Integer, Integer>();
    
    public void add(int number) {
        if (dict.containsKey(number)) {
            dict.put(number, dict.get(number) + 1);
        } else {
            dict.put(number, 1);
        }
    }

    public boolean find(int value) {
        for (Integer key : dict.keySet()) {
        <span style="color:#ff0000;">    if (value - key == key) {
                if (dict.get(key) >= 2) {
                    return true;
                </span>}
            } else if (dict.containsKey(value - key)) {
                return true;
            }
        }
    
        return false;
    }
    
    public static void main(String[] args) {
        TwoSumIII util = new TwoSumIII();
        util.add(1);
        util.add(3);
        util.add(5);
        
        System.out.println(util.find(4));
        System.out.println(util.find(7));
    }
    
}