[LeetCode] 339. Nested List Weight Sum

Problem

Given a nested list of integers, return the sum of all integers in the list weighted by their depth.

Each element is either an integer, or a list -- whose elements may also be integers or other lists.

Example 1:

Input: [[1,1],2,[1,1]]
Output: 10
Explanation: Four 1's at depth 2, one 2 at depth 1.
Example 2:

Input: [1,[4,[6]]]
Output: 27
Explanation: One 1 at depth 1, one 4 at depth 2, and one 6 at depth 3; 1 + 42 + 63 = 27.

Solution

/**
 *     // @return true if this NestedInteger holds a single integer, rather than a nested list.
 *     public boolean isInteger();
 *     // @return the single integer that this NestedInteger holds, if it holds a single integer
 *     public Integer getInteger();
 *     // @return the nested list that this NestedInteger holds, if it holds a nested list
 *     public List<NestedInteger> getList();
 */
class Solution {
    public int depthSum(List<NestedInteger> nestedList) {
        return helper(nestedList, 1);
    }
    private int helper(List<NestedInteger> list, int level) {
        int res = 0;
        for (NestedInteger item: list) {
            if (item.isInteger()) res += (level*item.getInteger());
            else {
                res += helper(item.getList(), level+1);
            }
        }
        return res;
    }
}