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# 0341. Flatten Nested List Iterator

<https://leetcode.com/problems/flatten-nested-list-iterator>

## Description

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]`.

## ac1: stack

DFS->Stack.

Memory efficient: get the value on the fly.

```java
public class NestedIterator implements Iterator<Integer> {
    Stack<NestedInteger> stack;
    NestedInteger curr;

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

    private void storeToStack(List<NestedInteger> nestedList) {
        for (int i = nestedList.size() - 1; i >= 0; i--) {
            stack.push(nestedList.get(i));
        }
    }

    private void prepareStack() {
        while(!stack.isEmpty() && !stack.peek().isInteger()) {
            storeToStack(stack.pop().getList());
        }
    }

    @Override
    public Integer next() {
        if (stack.isEmpty()) return null;
        Integer next = stack.pop().getInteger();

        prepareStack();

        return next;
    }

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

## ac2: Queue

Store all values in initiation, which is not memory efficient. Maybe not the correct iterator in the real world.

```java
/**
 * // 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> {
    private Queue<Integer> q;
    public NestedIterator(List<NestedInteger> nestedList) {
        q = new LinkedList<Integer>();
        flatten(nestedList);
    }

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

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

    private void flatten(List<NestedInteger> nestedList) {
        for (NestedInteger ni : nestedList) {
            if (ni.isInteger()) {
                q.offer(ni.getInteger());
            } else {
                flatten(ni.getList());
            }
        }
    }
}

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