<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>

#pragma GCC optimize("O3", "unroll-loops")
static const auto InitialOptimization = [](){
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    return 0;
}();

class Solution {
    size_t n;
    void backtracking(vector<vector<int>> &combinations, vector<int> &cur, size_t i, int target,
                      vector<int> &candidates) {
        if (target == 0) {
            combinations.push_back(cur);
            return;
        }
        if (i == n || target < 0) return;
        int next_num = 1;
        while (i + next_num < n and candidates[next_num + i] == candidates[i]) next_num++;
        backtracking(combinations, cur, i + next_num, target, candidates);
        for (int j = 1; j <= next_num; j++) {
            cur.push_back(candidates[i]);
            backtracking(combinations, cur, i + next_num, target - candidates[i] * j, candidates);
        }
        for (int j = 0; j < next_num; j++) cur.pop_back();
    }
public:
    vector<vector<int>> combinationSum2(vector<int> &candidates, int target) {
        n = candidates.size();
        vector<int> cur;
        vector<vector<int>> combinations;
        sort(candidates.begin(), candidates.end());
        backtracking(combinations, cur, 0, target, candidates);
        return combinations;
    }
};
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