Leetcode341-flattenNestedListIterator

Description

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

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].

Solution

/**
 * // 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 NestedIterator implements Iterator<Integer> {
    
    LinkedList<Integer> q;

    public NestedIterator(List<NestedInteger> nestedList) {
        q = new LinkedList<>();
        helper(nestedList);
    }
    
    private void helper(List<NestedInteger> list){
        if (list == null) return;
        
        for (NestedInteger it: list){
            if (it.isInteger()) q.offer(it.getInteger());
            else helper(it.getList());
        }
    }

    @Override
    public Integer next() {
        if (hasNext()) return q.poll();
        else return null;
    }

    @Override
    public boolean hasNext() {
        if (!q.isEmpty()) return true;
        return false;
    }
}

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

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