【刷题-LeetCode】216. Combination Sum III
- Combination Sum III
Find all possible combinations of k numbers that add up to a number n, given that only numbers from 1 to 9 can be used and each combination should be a unique set of numbers.
Note:
- All numbers will be positive integers.
- The solution set must not contain duplicate combinations.
Example 1:
Input: k = 3, n = 7
Output: [[1,2,4]]
Example 2:
Input: k = 3, n = 9
Output: [[1,2,6], [1,3,5], [2,3,4]]
解 定义函数f(l, r, n, target)
,表示在区间[l, r]之间和为target的n个数,则:
\[f(l, r, n, target) = \bigcup_{i=l}^r f(i+1, r, n-1, target - i)
\]
当n=2时,直接采用双指针搜索
class Solution {
public:
vector<vector<int>> combinationSum3(int k, int n) {
vector<bool>nums(10, true);
return comb(1, 9, k, n);
}
vector<vector<int>> comb(int l, int r, int n, int target){
vector<vector<int>>ans;
if(n == 2){
int i = l, j = r;
while(i < j){
int tmp = i + j;
if(tmp == target){
ans.push_back({i,j});
i++;
j--;
}else if(tmp < target){
i++;
}else{
j--;
}
}
return ans;
}
for(int i = l; i <= r; ++i){
vector<vector<int>>tmp_ans = comb(i+1, r, n - 1, target - i);
if(tmp_ans.size() > 0){
for(auto &v : tmp_ans){
v.insert(v.begin(), i);
ans.push_back(v);
}
}
}
return ans;
}
};
作者:Vinson
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个性签名:只要想起一生中后悔的事,梅花便落满了南山
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