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));
}

}