two sum 攻

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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 falsepublic 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));}}

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