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two-sum-iii-data-structure-design

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Two Sum III - Data structure design – LeetCode 170

Problem

Description

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.

Example

add(1); add(3); add(5);
find(4) -> true
find(7) -> false

Answer

Original

Code

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class TwoSum {
public:
    void add(int number) {
        ++m[number];
    }
    bool find(int value) {
        for (auto a : m) {
            int t = value - a.first;
            if ((t != a.first && m.count(t)) || (t == a.first && a.second > 1)) {
                return true;
            }
        }
        return false;
    }
private:
    unordered_map<int, int> m;
};

思路

重点在于原本容纳就需要n个空间，于是索性使用unordered_map，空间复杂度$O(n)$，add操作时间复杂度$O(1)$，find操作时间复杂度$O(n)$。

Better

思路

还没看到更好的解法