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
设计一个two sum 的类。包含add和find两种操作方式。
1、使用HashMap和HashSet实现。
public class TwoSum {
Map<Integer,Integer> map = new HashMap<Integer,Integer>();
Set<Integer> set = new HashSet<Integer>();
// Add the number to an internal data structure.
public void add(int number) {
if (set.contains(number)){
map.put(number, 2);
}else {
set.add(number);
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 (int num : set){
if (num == value - num){
if (map.get(num) == 2){
return true;
}
} else if (set.contains(value - num)){
return true;
}
}
return false;
}
}
// Your TwoSum object will be instantiated and called as such:
// TwoSum twoSum = new TwoSum();
// twoSum.add(number);
// twoSum.find(value);
2、使用HashMap 和 List（也可以只使用一个HashMap，记录一或者2即可。）
public class TwoSum {
private List<Integer> list = new ArrayList<Integer>();
private Map<Integer, Integer> map = new HashMap<Integer, Integer>();
// Add the number to an internal data structure.
public void add(int number) {
if (map.containsKey(number)) map.put(number, map.get(number) + 1);
else {
map.put(number, 1);
list.add(number);
}
}
// Find if there exists any pair of numbers which sum is equal to the value.
public boolean find(int value) {
for (int i = 0; i < list.size(); i++){
int num1 = list.get(i), num2 = value - num1;
if ((num1 == num2 && map.get(num1) > 1) || (num1 != num2 && map.containsKey(num2))) return true;
}
return false;
}
}
// Your TwoSum object will be instantiated and called as such:
// TwoSum twoSum = new TwoSum();
// twoSum.add(number);
// twoSum.find(value);
3、为什么要使用一个List，因为在遍历操作的时候，List要比Map或者Set快得多，使用一个List可以用空间换取时间。