Sum of nested list of 'NestedInteger'

NestedIntegers1.java

public class Solution {
  public long sum(List<NestedInteger> list) {
    return sumRecurse(list, 0); // 0 being the depth 
  }
  
  public long sumRecurse(List<NestedInteger> list, int level) {
    if(list == null || list.size() == 0) {
      return 0; 
    }
    long sum = 0;
    for(NestedInteger element : list) {
      if(element.isInteger()) {
        sum = sum + element.getInteger() * level; 
      }else {
        sum = sum + sumRecurse(element.getList(), level + 1); 
      }
    }
    return sum;
  }
}