LeetCode-in-Java

341. Flatten Nested List Iterator

Medium

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:

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:

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:

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].

Solution

import com_github_leetcode.NestedInteger;
import java.util.ArrayDeque;
import java.util.Deque;
import java.util.Iterator;
import java.util.List;
import java.util.ListIterator;

/*
 * // 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 empty list if this NestedInteger holds a single integer
 *     public List<NestedInteger> getList();
 * }
 */
@SuppressWarnings("java:S2272")
public class NestedIterator implements Iterator<Integer> {
    private Deque<ListIterator<NestedInteger>> stack;
    private ListIterator<NestedInteger> it;
    private NestedInteger next;

    public NestedIterator(List<NestedInteger> nestedList) {
        stack = new ArrayDeque<>();
        it = nestedList.listIterator();
    }

    @Override
    public Integer next() {
        return next.getInteger();
    }

    @Override
    public boolean hasNext() {
        while (it.hasNext() || !stack.isEmpty()) {
            // end of list
            if (!it.hasNext()) {
                it = stack.pop();
                continue;
            }
            NestedInteger ni = it.next();
            if (ni.isInteger()) {
                next = ni;
                return true;
            } else {
                stack.push(it);
                it = ni.getList().listIterator();
            }
        }
        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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