<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 {
public:
    vector<vector<int>> func(int freq[], int target, int val ){
        vector<vector<int>> ans;
        ans.clear();
        if(val > target){
            ans.clear();
            return ans;
        }
        if(freq[val] == 0) return func(freq, target, val+1);
        for(int i = 0; i<=freq[val]; i++)
        {
            if(i*val > target) break;
            else{
                bool flag = true;
                for(auto ele : func(freq, target-i*val, val+1)){
                    // each ele is a vector.
                    flag = false;
                    for(int j = 0; j<i; j++) ele.push_back(val);
                    ans.push_back(ele);
                }
                if(i*val == target and flag){
                    vector<int> temp;
                    for(int j = 0; j<i; j++) temp.push_back(val);
                    ans.push_back(temp);
                }
            }
        }

        return ans;
    }
    vector<vector<int>> combinationSum2(vector<int>& candidates, int target) {
        int freq[31] = {0};
        for(auto ele : candidates){
            if(ele <= 30 )freq[ele]++;
        }
        // for(int i = 1; i<=30; i++) cout << i << " " << freq[i] <<"\n";
        return func(freq, target, 1);

    }
};
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