LeetCode-Flatten Nested List Iterator

LiBlog發表於2016-08-05
LeetCode

Given a nested list of integers, implement an iterator to flatten it.

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]],

By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,1,2,1,1].

Example 2:
Given the list [1,[4,[6]]],

By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,4,6].

Analysis:

We use recursive iterator. If the next element is a list, we create another iterator to traverse it. One key point here is: either there is next element or the hasNext() function returns false.

After fetch the curent element (return of Next()), we need to move the pointer to the next element. When moving, we need to (1) determine whether there is next element; (2) skip all empty lists.

Solution:

 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 NestedIterator implements Iterator<Integer> {
19     
20     List<NestedInteger> list;
21     NestedIterator curIterator;
22     int nextIndex;
23     boolean isNestedList;
24     boolean hasNext;
25 
26     public NestedIterator(List<NestedInteger> nestedList) {
27         list = nestedList;
28         nextIndex = -1;
29         moveToNext();
30     }
31 
32     @Override
33     public Integer next() {
34         Integer nextVal= null;
35         if (isNestedList){
36             nextVal = curIterator.next();
37             if (!curIterator.hasNext()) moveToNext();
38         } else {
39             nextVal = list.get(nextIndex).getInteger();
40             moveToNext();
41         }
42         return nextVal;
43     }
44 
45     @Override
46     public boolean hasNext() {
47         return hasNext;
48     }
49     
50     public void moveToNext(){
51         while (true){
52             nextIndex++;
53             if (nextIndex>=list.size()){
54                 hasNext = false;
55                 return;
56             }
57         
58             isNestedList = !list.get(nextIndex).isInteger();
59             if (isNestedList){
60                 curIterator = new NestedIterator(list.get(nextIndex).getList());
61                 if (!curIterator.hasNext()) continue;
62             } else curIterator = null;
63             hasNext = true;
64             return;
65         }
66     }
67 }
68 
69 /**
70  * Your NestedIterator object will be instantiated and called as such:
71  * NestedIterator i = new NestedIterator(nestedList);
72  * while (i.hasNext()) v[f()] = i.next();
73  */

 

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