339 Nested List Weight Sum

339. Nested List Weight Sum

1. Question

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]]], return27. (one 1 at depth 1, one 4 at depth 2, and one 6 at depth 3; 1 + 4*2 + 6*3 = 27)

2. Implementation

(1) DFS
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/**
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* // This is the interface that allows for creating nested lists.
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* // You should not implement it, or speculate about its implementation
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* public interface NestedInteger {
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* // Constructor initializes an empty nested list.
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* public NestedInteger();
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*
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* // Constructor initializes a single integer.
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* public NestedInteger(int value);
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*
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* // @return true if this NestedInteger holds a single integer, rather than a nested list.
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* public boolean isInteger();
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*
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* // @return the single integer that this NestedInteger holds, if it holds a single integer
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* // Return null if this NestedInteger holds a nested list
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* public Integer getInteger();
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*
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* // Set this NestedInteger to hold a single integer.
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* public void setInteger(int value);
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*
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* // Set this NestedInteger to hold a nested list and adds a nested integer to it.
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* public void add(NestedInteger ni);
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*
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* // @return the nested list that this NestedInteger holds, if it holds a nested list
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* // Return null if this NestedInteger holds a single integer
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* public List<NestedInteger> getList();
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* }
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*/
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class Solution {
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public int depthSum(List<NestedInteger> nestedList) {
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if (nestedList == null || nestedList.size() == 0) {
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return 0;
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}
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return getDepthSum(nestedList, 1);
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}
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public int getDepthSum(List<NestedInteger> nestedList, int depth) {
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int res = 0;
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for (NestedInteger ni : nestedList) {
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res += ni.isInteger() ? ni.getInteger() * depth : getDepthSum(ni.getList(), depth + 1);
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}
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return res;
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}
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}
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3. Time & Space Complexity

DFS: 时间复杂度O(n), n为nestedList的所有数的个数, 空间复杂度O(k), k为nestedList中的深度