<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>> combinationSum2(vector<int>& candidates, int target) {
        vector<int> new_cands;
        unordered_map<int, int> lookup;
        for_each(candidates.begin(), candidates.end(), [&](const int &val) {
            if (!lookup.contains(val)) {
                new_cands.emplace_back(val);
                lookup[val] = 0;
            }
            lookup[val]++;
        });

        int total{0};
        vector<int> curr;
        vector<vector<int>> result;

        function<void(size_t)> helper = [&](const size_t index) {
            if (total == target) {
                result.emplace_back(curr);
                return;
            }
            if (index >= new_cands.size() || total > target)
                return;

            curr.emplace_back(new_cands[index]);
            total += new_cands[index];
            if (lookup[new_cands[index]]-- > 0) {
                helper(index);
            }

            curr.pop_back();
            total -= new_cands[index];
            lookup[new_cands[index]]++;
            helper(index + 1);
        };

        helper(0);
        return result;
    }
};
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