Question

Formatted question description: https://leetcode.ca/all/341.html

 341	Flatten Nested List Iterator

 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:

 Input: [[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:

 Input: [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].

 @tag-stack

Algorithm

2 possible approaches: DFS and iteration.

Iteration generally requires the use of the stack to assist traversal, due to the last-in first-out characteristic of the stack.

When we traverse the vector, push the object onto the stack from back to front, then the first object will be the first one to be taken out for processing after being pushed onto the stack.

The hasNext() function needs to traverse the stack and process it,

If the top element of the stack is an integer, return true directly,
If not, then remove the top element of the stack, and start traversing the retrieved list, or push it back to the stack

The loop stop condition is that the stack is empty and false is returned.

Code

Java

public class Flatten_Nested_List_Iterator {

    // ref: https://leetcode.com/problems/flatten-nested-list-iterator/discuss/80147/Simple-Java-solution-using-a-stack-with-explanation
    public class NestedIterator_stack implements Iterator<Integer> {

        Deque<NestedInteger> stack = new ArrayDeque<>();
        public NestedIterator(List<NestedInteger> nestedList) {
            prepareStack(nestedList);
        }

        @Override
        public Integer next() {
            if (!hasNext()) {
                return null;
            }
            return stack.pop().getInteger();
        }

        @Override
        public boolean hasNext() {
            while (!stack.isEmpty() && !stack.peek().isInteger()) {
                List<NestedInteger> list = stack.pop().getList();
                prepareStack(list);
            }
            return !stack.isEmpty();
        }
        
        private void prepareStack(List<NestedInteger> nestedList) {
            for (int i = nestedList.size() - 1; i >= 0; i--) {
                stack.push(nestedList.get(i));
            }
        }
    }


    public class NestedIterator implements Iterator<Integer> {
        final Queue<Integer> queue = new LinkedList<Integer>();

        public NestedIterator(List<NestedInteger> nestedList) {
            if (nestedList == null || nestedList.size() == 0) {
                return;
            }
            dfs(nestedList);
        }

        private void dfs(List<NestedInteger> list) {
            for (NestedInteger nestedInteger : list) {
                if (nestedInteger.isInteger()) {
                    queue.offer(nestedInteger.getInteger());
                } else {
                    dfs(nestedInteger.getList());
                }
            }
        }

        @Override
        public Integer next() {
            if (!queue.isEmpty()) {
                return queue.poll();
            }
            return null;
        }

        @Override
        public boolean hasNext() {
            return !queue.isEmpty();
        }
    }


    /**
     * Your NestedIterator object will be instantiated and called as such:
     * NestedIterator i = new NestedIterator(nestedList);
     * while (i.hasNext()) v[f()] = i.next();
     */


    // 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();
    }

}

// OJ: https://leetcode.com/problems/flatten-nested-list-iterator/
// Time: O(1) amortized
// Space: O(D) where D is the max depth of the list
class NestedIterator {
    typedef vector<NestedInteger>::iterator iter;
    stack<pair<iter, iter>> s;
    void goToInteger() {
        while (s.size()) {
            if (s.top().first == s.top().second) {
                s.pop();
                if (s.size()) s.top().first++;
            } else if (s.top().first->isInteger()) break;
            else {
                auto &list = s.top().first->getList();
                s.emplace(list.begin(), list.end());
            }
        }
    }
public:
    NestedIterator(vector<NestedInteger> &list) {
        s.emplace(list.begin(), list.end());
        goToInteger();
    }
    
    int next() {
        int val = s.top().first->getInteger();
        s.top().first++;
        goToInteger();
        return val;
    }
    
    bool hasNext() {
        return s.size();
    }
};

# """
# This is the interface that allows for creating nested lists.
# You should not implement it, or speculate about its implementation
# """
# class NestedInteger(object):
#    def isInteger(self):
#        """
#        @return True if this NestedInteger holds a single integer, rather than a nested list.
#        :rtype bool
#        """
#
#    def getInteger(self):
#        """
#        @return the single integer that this NestedInteger holds, if it holds a single integer
#        Return None if this NestedInteger holds a nested list
#        :rtype int
#        """
#
#    def getList(self):
#        """
#        @return the nested list that this NestedInteger holds, if it holds a nested list
#        Return None if this NestedInteger holds a single integer
#        :rtype List[NestedInteger]
#        """
from collections import deque


class NestedIterator(object):

  def __init__(self, nestedList):
    """
    Initialize your data structure here.
    :type nestedList: List[NestedInteger]
    """
    self.stack = deque(nestedList[::-1])
    self.value = None

  def next(self):
    """
    :rtype: int
    """
    self.hasNext()
    ret = self.value
    self.value = None
    return ret

  def hasNext(self):
    """
    :rtype: bool
    """
    if self.value is not None:
      return True

    stack = self.stack
    while stack:
      top = stack.pop()
      if top.isInteger():
        self.value = top.getInteger()
        return True
      else:
        stack.extend(top.getList()[::-1])
    return False

# Your NestedIterator object will be instantiated and called as such:
# i, v = NestedIterator(nestedList), []
# while i.hasNext(): v.append(i.next())

341 - Flatten Nested List Iterator

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