嵌套列表的加权和

http://www.lintcode.com/zh-cn/problem/nested-list-weight-sum/

import java.util.List;

/**
 * // This is the interface that allows for creating nested lists.
 * // You should not implement it, or speculate about its implementation
 * public interface NestedInteger {
 * 

 * // @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
 * // Return null if this NestedInteger holds a nested list
 * public Integer getInteger();
 * 

 * // @return the nested list that this NestedInteger holds,
 * // if it holds a nested list
 * // Return null if this NestedInteger holds a single integer
 * public List getList();
 * }
 */
public class Solution {
    private int sum = 0;

    public int depthSum(List nestedList) {
        // Write your code here
        treeWalk(nestedList, 1);
        return sum;
    }

    private void treeWalk(List nestedList, int depth) {
        if (nestedList == null) {
            return;
        }

        for (int i = 0; i < nestedList.size(); i++) {
            NestedInteger nestedInteger = nestedList.get(i);
            if (nestedInteger.isInteger()) {
                sum += nestedInteger.getInteger() * depth;
            } else {
                treeWalk(nestedInteger.getList(), depth + 1);
            }
        }
    }
}