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)

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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

/**
 * // 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<NestedInteger> getList();
 * }
 */
public class Solution 
{
    public int depthSum(List<NestedInteger> nestedList) 
    {
        int res = 0;
        
        // iterate list
        for(int i=0; i<nestedList.size(); i++)
            res += getSum(nestedList.get(i), 1);
        
        return res;
    }
    
    public int getSum(NestedInteger ele, int depth)
    {
        int sum = 0;
        
        // if ele is integer, return its value * depth;
        if(ele.isInteger())
            return depth * ele.getInteger();
        
        // if ele is a list, iterate list recursively call function;
        for(int i=0; i<ele.getList().size(); i++)
            sum += getSum(ele.getList().get(i), depth + 1);
        
        
        return sum;
    }
}

参考资料：

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

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