<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:
    vector<vector<int>> res;
public:
    vector<vector<int>> combinationSum2(vector<int>& candidates, int target) {
        
        sort(candidates.begin(), candidates.end());
        int last = 0;

        for (int i = 0; i < candidates.size(); ++i) {
            if (candidates[i] <= target && candidates[i] != last) {
                // include it and see if the rest works
                vector<int> comb{candidates[i]};
                findCombinations(candidates, comb, i+1, target-candidates[i]);
                last = candidates[i];
            }
        }

        return res;
    }

    void findCombinations(vector<int>& candidates, vector<int> comb, int index, int target) {
    
        if (target < 0) return;
        else if (target == 0) res.push_back(comb);
        else {
            if (index != candidates.size()) {
                int original_index = index;
                // 1. try moving on by skipping all same elements
                while (index < candidates.size()-1 && candidates[index] == candidates[index+1])
                    ++index;
                findCombinations(candidates, comb, index+1, target);
                index = original_index;

                if (candidates[index] <= target) {
                    // 2. try using our element and move on
                    comb.push_back(candidates[index]);
                    findCombinations(candidates, comb, index+1, target-candidates[index]);
                }
            }
        }
    }
};
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