<p>Given a collection of candidate numbers (<code>candidates</code>) and a target number (<code>target</code>), find all unique combinations in <code>candidates</code>&nbsp;where the candidate numbers sum to <code>target</code>.</p>

<p>Each number in <code>candidates</code>&nbsp;may only be used <strong>once</strong> in the combination.</p>

<p><strong>Note:</strong>&nbsp;The solution set must not contain duplicate combinations.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> candidates = [10,1,2,7,6,1,5], target = 8
<strong>Output:</strong> 
[
[1,1,6],
[1,2,5],
[1,7],
[2,6]
]
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> candidates = [2,5,2,1,2], target = 5
<strong>Output:</strong> 
[
[1,2,2],
[5]
]
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;=&nbsp;candidates.length &lt;= 100</code></li>
	<li><code>1 &lt;=&nbsp;candidates[i] &lt;= 50</code></li>
	<li><code>1 &lt;= target &lt;= 30</code></li>
</ul>

class combinationSolver {
private:
    const vector<int> candidates;
    unordered_map<int, int> candidates_freq;

    unordered_map<int, int> freq_map(const vector<int>& candidates) const {
        unordered_map<int, int> ans;
        for (const int& candidate : candidates) {
            if (ans.find(candidate) == ans.end()) {
                ans.emplace(candidate, 1);
            } else {
                ++ans[candidate];
            }
        }
        return ans;
    }

    vector<int> uniques(const vector<int>& v) const {
        unordered_set<int> st;
        for (const int& i : v) {
            st.insert(i);
        }

        return vector<int>(st.begin(), st.end());
    }

    vector<vector<int>> solve(const int& target, const int& i) {
        vector<vector<int>> ans;

        if (i == candidates.size()) {
            return ans;
        }

        if (candidates[i] == target && candidates_freq.at(candidates[i])) {
            ans.emplace_back(initializer_list<int>{candidates[i]});
        }

        if (candidates[i] < target && candidates_freq.at(candidates[i])) {
            --candidates_freq[candidates[i]];
            for (vector<int>& v : solve(target - candidates[i], i)) {
                v.push_back(candidates[i]);
                ans.push_back(v);
            }
            ++candidates_freq[candidates[i]];
        }

        for (vector<int>& v : solve(target, i + 1)) {
            ans.push_back(v);
        }

        return ans;
    };

public:
    combinationSolver(const vector<int>& candidates)
        : candidates_freq(freq_map(candidates)),
          candidates(uniques(candidates)) {}

    vector<vector<int>> solve(const int& target) { return solve(target, 0); }
};

class Solution {
public:
    vector<vector<int>> combinationSum2(const vector<int>& candidates,
                                        const int& target) const {
        return combinationSolver(candidates).solve(target);
    }
};
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