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# 341. 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.

### Constraints

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

### Approach

### Links

* GeeksforGeeks
* [Leetcode](https://leetcode.com/problems/flatten-nested-list-iterator/)
* [ProgramCreek](https://www.programcreek.com/2014/05/leetcode-flatten-nested-list-iterator-java/)
* YouTube

### **Examples**

{% tabs %}
{% tab title="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].
{% endtab %}

{% tab title="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].
{% endtab %}
{% endtabs %}

### **Solutions**

{% tabs %}
{% tab title="NestedInteger" %}

```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 empty list if this NestedInteger holds a single integer
	public List<NestedInteger> getList();
}
```

{% endtab %}

{% tab title="Solution 1" %}

```java
/**
 * Time complexity : 
 * Space complexity : 
 */

public class NestedIterator implements Iterator<Integer> {
    private Stack<NestedInteger> stack = new Stack();

    public NestedIterator(List<NestedInteger> nestedList) {
        if(nestedList == null || nestedList.isEmpty()) {
            return;
        }
        for(int i = nestedList.size()-1; i >= 0; i--) {
            stack.push(nestedList.get(i));
        }
    }

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

    @Override
    public boolean hasNext() {
        while(!stack.isEmpty()) {
            NestedInteger top = stack.peek();
            if(top.isInteger()) {
                return true;
            } else {
                stack.pop();
                List<NestedInteger> topList = top.getList();
                for(int i = topList.size()-1; i >= 0; i--) {
                    stack.push(topList.get(i));
                }
            }
        }
        return false;
    }
}

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

{% endtab %}
{% endtabs %}

### **Follow up**

*