[LeetCode170]Two Sum III - Data structure design

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
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这种设计，首先要确定用什么data structure. 肯定不会用vector, because it doesn’t have the constant access time for finding each element. 所以我想到了set, 因为可能有multiple duplicates, 所以用

unordered_multiset<int> nums

.

本来想直接find结果超时了, 为什么？ 因为题目申明是TWO SUM,至少两个，并不是光找到就可以， 比如：add(2), add(3), add(4), find(4). when we search in nums, current key is 2, and what we have to find is another 2, however the set doesnt contain another 2, therefore, we should return false.如果只是单一的find 就会return true.

class TwoSum {
public:
unordered_multiset<int> nums;
// Add the number to an internal data structure.
void add(int number) {
nums.insert(number);
}

// Find if there exists any pair of numbers which sum is equal to the value.
bool find(int value) {
for(int i : nums){
int count = (i == value - i) ? 1 : 0;// if i = 2, value = 4. only if nums contains at least two 2's we can return true
// TWO SUM please!
if (nums.count(value - i) > count) {
return true;
}
}
return false;

}
};
// Your TwoSum object will be instantiated and called as such:
// TwoSum twoSum;
// twoSum.add(number);
// twoSum.find(value);