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
/**
* Your TwoSum object will be instantiated and called as such:
* TwoSum obj = new TwoSum();
* obj.add(number);
* boolean param_2 = obj.find(value);
*/
此题是一个设计题，和之前的两个two sum题目不同的地方是，此题不是给数组，而是给了add，find方法，此题有个小细节需要考虑，实际操作中究竟是add的操作多还是find的操作多？因此对应了以下几种做法：1.add的操作多的时候，可以用hashmap来存储数据，key保存加的值，value保存同一个key值出现的次数。然后迭代哈希表键值来检查是否和等于指定值。代码如下：
public class TwoSum {
Map<Integer,Integer> map;
/** Initialize your data structure here. */
public TwoSum() {
map = new HashMap<Integer,Integer>();
}
/** Add the number to an internal data structure.. */
public void add(int number) {
map.put(number,map.containsKey(number)?map.get(number)+1:1);
}
/** 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 a = entry.getKey();
int b = value-a;
if((a==b&&map.get(a)>1)||a!=b&&map.containsKey(b)){
return true;
}
}
return false;
}
}
2.接下来考虑find多的情况,即把add的数都两个两个加起来，存储到hashset里面，然而这么做相当于是用空间去换了时间，最后给出了TLE，该算法不可行。代码如下：
public class TwoSum {
Set<Integer> num;
Set<Integer> sum;
/** Initialize your data structure here. */
public TwoSum() {
num = new HashSet<Integer>();
sum = new HashSet<Integer>();
}
/** Add the number to an internal data structure.. */
public void add(int number) {
if(!num.contains(number)){
Iterator<Integer> iter = num.iterator();
while(iter.hasNext()){
sum.add(iter.next()+number);
}
num.add(number);
}else{
sum.add(2*number);
}
}
/** Find if there exists any pair of numbers which sum is equal to the value. */
public boolean find(int value) {
return sum.contains(value);
}
}
此题可以学到，hashset和hashmap都可以用iterator进行迭代（Iterator<Integer> iter = hashmap.keySet().iterator()），也可以手动对hashmap进行迭代，如Map.Entry<a,b> entry;以后还会有keySet（），valueSet（）等等