<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 Solution {
private:
    void helper(vector<int>& candidates,int i, int target,vector<int>& freq, vector<int>& ds, vector<vector<int>> & ans) {
        if(0 == target) {
            ans.push_back(ds);
            return;
        }
        if(i >= freq.size() || target < 0) return;
        
        helper(candidates, i+1, target,freq, ds,ans);

        if(freq[i]) {
            ds.push_back(i);
            freq[i]--;
            helper(candidates, i, target - i, freq, ds,ans);
            freq[i]++;
            ds.pop_back();
        }
    }
public:
    vector<vector<int>> combinationSum2(vector<int>& candidates, int target) {
        vector<vector<int>> ans;
        vector<int> ds;
        vector<int> freq(51,0);
        for (auto e : candidates) {
            freq[e]++;
        }
        helper(candidates, 0, target,freq, ds,ans);
        return ans;
    }
};
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