341. Flatten Nested List Iterator

Posted on Edited on In

341. Flatten Nested List Iterator

Description

Difficulty: Medium

Related Topics: Stack, Tree, Depth-First Search, Design, Queue, Iterator

You are given a nested list of integers nestedList. Each element is either an integer or a list whose elements may also be integers or other lists. Implement an iterator to flatten it.

Implement the NestedIterator class:

  • NestedIterator(List<NestedInteger> nestedList) Initializes the iterator with the nested list nestedList.
  • int next() Returns the next integer in the nested list.
  • boolean hasNext() Returns true if there are still some integers in the nested list and false otherwise.

Your code will be tested with the following pseudocode:

1
2
3
4
5
initialize iterator with nestedList
res = []
while iterator.hasNext()
    append iterator.next() to the end of res
return res

If res matches the expected flattened list, then your code will be judged as correct.

Example 1:

1
2
3
Input: nestedList = [[1,1],2,[1,1]]
Output: [1,1,2,1,1]
Explanation: By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,1,2,1,1].

Example 2:

1
2
3
Input: nestedList = [1,[4,[6]]]
Output: [1,4,6]
Explanation: By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,4,6].

Constraints:

  • 1 <= nestedList.length <= 500
  • The values of the integers in the nested list is in the range [-106, 106].

Hints/Notes

  • we can do the traverse and generate the list during construction, or we can do the lazy iterator, which only check if there’s next item when calling hasNext()
  • lazy iterator is better, and it needs us to change the vector when calling hasNext()
  • deque provides the best performance, since we don’t care about random access and deque
    allocates memory in large blocks

Solution

Language: C++

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
/**
 * // This is the interface that allows for creating nested lists.
 * // You should not implement it, or speculate about its implementation
 * class NestedInteger {
 *   public:
 *     // Return true if this NestedInteger holds a single integer, rather than a nested list.
 *     bool isInteger() const;
 *
 *     // Return the single integer that this NestedInteger holds, if it holds a single integer
 *     // The result is undefined if this NestedInteger holds a nested list
 *     int getInteger() const;
 *
 *     // Return the nested list that this NestedInteger holds, if it holds a nested list
 *     // The result is undefined if this NestedInteger holds a single integer
 *     const vector<NestedInteger> &getList() const;
 * };
 */

class NestedIterator {
public:
    deque<NestedInteger> res;

    NestedIterator(vector<NestedInteger> &nestedList) {
        // res = new vector<NestedInteger>(nestedList);
        res = deque<NestedInteger>(nestedList.begin(), nestedList.end());
    }

    int next() {
        int val = res.front().getInteger();
        res.pop_front();
        return val;
    }

    bool hasNext() {
        while (!res.empty() && res.front().isInteger() == false) {
            vector<NestedInteger> frontList = res.front().getList();
            res.pop_front();
            for (int i = frontList.size() - 1; i >= 0; i--) {
                res.push_front(frontList[i]);
            }
        }
        return !res.empty();
    }
};

/**
 * Your NestedIterator object will be instantiated and called as such:
 * NestedIterator i(nestedList);
 * while (i.hasNext()) cout << i.next();
 */