Leetcode | Combination Sum I && II

 Combination Sum I 

Given a set of candidate numbers (C) and a target number (T), find all unique combinations in C where the candidate numbers sums to T.

The same repeated number may be chosen from C unlimited number of times.

Note:
All numbers (including target) will be positive integers.
Elements in a combination (a1, a2, … , ak) must be in non-descending order. (ie, a1 ≤ a2 ≤ … ≤ ak).
The solution set must not contain duplicate combinations.
For example, given candidate set 2,3,6,7 and target 7,
A solution set is:
[7]
[2, 2, 3]

先排序，然后每次从当前位置开始找下一个数。去下重，不过貌似leetcode的testcases没有针对这个。

class Solution {
public:
    vector<vector<int> > combinationSum(vector<int> &candidates, int target) {
        vector<vector<int> > ans;
        if (candidates.empty()) return ans;
        vector<int> tmp;
        sort(candidates.begin(), candidates.end());
        recurse(candidates, 0, target, tmp, ans);
        return ans;
    }
    void recurse(vector<int> &candidates, int start, int target, vector<int> &tmp, vector<vector<int> > &ans) {
        if (target < 0) return;
        if (target == 0) {
            ans.push_back(tmp);
            return;
        }
        
        for (int i = start; i < candidates.size(); i++) {
            tmp.push_back(candidates[i]);
            recurse(candidates, i, target - candidates[i], tmp, ans);
            tmp.pop_back();
        }
    }
};

 

 Combination Sum II

Given a collection of candidate numbers (C) and a target number (T), find all unique combinations in C where the candidate numbers sums to T.

Each number in C may only be used once in the combination.

Note:
All numbers (including target) will be positive integers.
Elements in a combination (a1, a2, … , ak) must be in non-descending order. (ie, a1 ≤ a2 ≤ … ≤ ak).
The solution set must not contain duplicate combinations.
For example, given candidate set 10,1,2,7,6,1,5 and target 8,
A solution set is:
[1, 7]
[1, 2, 5]
[2, 6]
[1, 1, 6]

这道题和上面类似。但是每个数只能使用一次。如果是重复数，在每个位置上用的应该是第一个数，这样才能保证这个重复数可以重复使用。

比如[1,1,2,3]，如果每个位置用的是重复数的最后一个，那么就生成不了[1,1,2]了。

class Solution {
public:
    vector<vector<int> > combinationSum2(vector<int> &num, int target) {
        sort(num.begin(), num.end());
        vector<int> r;
        recursive(num, target, 0, r);
        return ret;
    }
    
    void recursive(vector<int> &num, int target, int start, vector<int> &r) {
        if (target <= 0) {
            if (0 == target) ret.push_back(r);
            return;
        }
        
        for (int i = start; i < num.size(); ++i) {
            if (i > start && num[i] == num[i - 1]) continue;
            r.push_back(num[i]);
            recursive(num, target - num[i], i + 1, r);
            r.pop_back();
        }
    }
private:
    vector<vector<int> > ret;
};