有人能解释一下下面的陈述吗

if (dict.contains(sum-k)) count+= dict(sum-k)
            dict(sum) += 1

实际代码：

object Solution {
    def subarraySum(nums: Array[Int], k: Int): Int = {
        if (nums isEmpty) return 0

    val dict = scala.collection.mutable.Map[Int, Int]().withDefaultValue(0)
    var count = 0

    nums.scanLeft(0)(_+_).foreach { sum => 
      **if (dict.contains(sum-k)) count+= dict(sum-k)
        dict(sum) += 1**
    }

    count
}

}