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Last active December 31, 2015 07:58
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// LeetCode, 4Sum
// 用一个 hashmap 先缓存两个数的和
// 时间复杂度O(n^2),空间复杂度O(n^2)
// @author 龚陆安(http://weibo.com/luangong)
class Solution {
public:
vector<vector<int>> fourSum(vector<int>& num, int target) {
vector<vector<int>> result;
if (num.size() < 4) return result;
sort(num.begin(), num.end());
unordered_multimap<int, pair<int, int>> cache;
for (int i = 0; i + 1 < num.size(); ++i)
for (int j = i + 1; j < num.size(); ++j)
cache.insert(make_pair(num[i] + num[j], make_pair(i, j)));
for (auto i = cache.begin(); i != cache.end(); ++i) {
int x = target - i->first;
auto range = cache.equal_range(x);
for (auto j = range.first; j != range.second; ++j) {
auto a = i->second.first;
auto b = i->second.second;
auto c = j->second.first;
auto d = j->second.second;
if (a != c && a != d && b != c && b != d) {
vector<int> vec = { num[a], num[b], num[c], num[d] };
sort(vec.begin(), vec.end());
result.push_back(vec);
}
}
}
sort(result.begin(), result.end());
result.erase(unique(result.begin(), result.end()), result.end());
return result;
}
};
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