LeetCode 339. Nested List Weight Sum (嵌套列表重和)$

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:
Given the list [[1,1],2,[1,1]], return 10. (four 1's at depth 2, one 2 at depth 1)

Example 2:
Given the list [1,[4,[6]]], return 27. (one 1 at depth 1, one 4 at depth 2, and one 6 at depth 3; 1 + 4*2 + 6*3 = 27)

 


题目标签:Depth First Search

  这道题目给了我们一个嵌套的list,在这个list里,每一个element可以是一个integer,又可以是一个list,这个list里还可以继续有list,可以无限套。所以要用到depth first search。如果能走到最里面一层呢,要利用recursive function,一层一层递归下去直到它是一个integer了,就可以返回了。所以需要另外一个function getSum。首先iterate nestedList, 把每一个element 加起来。 为了得到这个element的值,我们要把它代入getSum function, 如果这个element 是integer 直接return。 如果这个element 是一个list,那么利用相同的方法,设一个sum 把它每一个element 加起来,为了得到每一个element,把每一个element代入getSum,记得这里要把depth + 1。因为我们代入了下一层depth。

 

Java Solution:

Runtime beats 6.27% 

完成日期:07/09/2017

关键词:Depth First Search

关键点:利用递归function来实现depth first search

 

 1 /**
 2  * // This is the interface that allows for creating nested lists.
 3  * // You should not implement it, or speculate about its implementation
 4  * public interface NestedInteger {
 5  *
 6  *     // @return true if this NestedInteger holds a single integer, rather than a nested list.
 7  *     public boolean isInteger();
 8  *
 9  *     // @return the single integer that this NestedInteger holds, if it holds a single integer
10  *     // Return null if this NestedInteger holds a nested list
11  *     public Integer getInteger();
12  *
13  *     // @return the nested list that this NestedInteger holds, if it holds a nested list
14  *     // Return null if this NestedInteger holds a single integer
15  *     public List<NestedInteger> getList();
16  * }
17  */
18 public class Solution 
19 {
20     public int depthSum(List<NestedInteger> nestedList) 
21     {
22         int res = 0;
23         
24         // iterate list
25         for(int i=0; i<nestedList.size(); i++)
26             res += getSum(nestedList.get(i), 1);
27         
28         return res;
29     }
30     
31     public int getSum(NestedInteger ele, int depth)
32     {
33         int sum = 0;
34         
35         // if ele is integer, return its value * depth;
36         if(ele.isInteger())
37             return depth * ele.getInteger();
38         
39         // if ele is a list, iterate list recursively call function;
40         for(int i=0; i<ele.getList().size(); i++)
41             sum += getSum(ele.getList().get(i), depth + 1);
42         
43         
44         return sum;
45     }
46 }

参考资料:

http://www.cnblogs.com/grandyang/p/5340305.html

 

LeetCode 算法题目列表 - LeetCode Algorithms Questions List

posted @ 2017-07-10 01:27  Jimmy_Cheng  阅读(311)  评论(0编辑  收藏  举报