377. Combination Sum IV

Given an integer array with all positive numbers and no duplicates, find the number of possible combinations that add up to a positive integer target.

Example:

nums = [1, 2, 3]
target = 4

The possible combination ways are:
(1, 1, 1, 1)
(1, 1, 2)
(1, 2, 1)
(1, 3)
(2, 1, 1)
(2, 2)
(3, 1)

Note that different sequences are counted as different combinations.

Therefore the output is 7.

统计个数，如果还是用上3题的方法，超时了。

class Solution {
public:
    int finalres ;
    int combinationSum4(vector<int>& candidates, int target) {
        help(0,candidates,target);
        return finalres;
        
    }
    void help(int cursum,vector<int>& candidates,int target){
        if(cursum==target)
             finalres ++;
        if(cursum>target)
            return;
        for(int i = 0;i<candidates.size();i++){
            help(cursum,candidates,target-candidates[i]);
        }
    }
};

只是统计个数的话，其实是不用把每个组合都排出来，而且这个题并不是真正意义的排列问题

因为每个数字可以用无限次。

 c++ leetcode有bug 所以用java

class Solution {
   public int combinationSum4(int[] nums, int target) {
    int[] dp = new int[target + 1];
    dp[0] = 1;
    for (int i = 1; i <=target; i++) 
        for (int j = 0; j < nums.length; j++) 
            if (i - nums[j] >= 0) 
                dp[i] += dp[i - nums[j]];   
    return dp[target];
    }
}