Leetcode: 377. Combination Sum IV

func combinationSum4(nums []int, target int) int {
    // dynamic programming, combination cache by target
    targetCounts := map[int]int{}
    
    var combinationCount func(int) int
    combinationCount = func(target int) int {
        if target <= 0 {
            return 0
        }
        
        count, ok := targetCounts[target]
        if ok {
            return count
        }
        
        for _, num := range nums {
            if num == target {
                count++
            } else {
                count += combinationCount(target - num)
            }
        }
        
        targetCounts[target] = count
        return count
    }
    
    return combinationCount(target)
    
}