# 341.Flatten-Nested-List-Iterator

## ้ข็ฎๅฐๅ

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

## ้ข็ฎๆ่ฟฐ

``````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].``````

## ไปฃ็ 

### Approach #1 Recursion

``````/**
* // 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 List<Integer> integers = new ArrayList<Integer>();
private int position = 0;

public NestedIterator(List<NestedInteger> nestedList) {
flattenList(nestedList);
}

private void flattenList(List<NestedInteger> nestedList) {
for (NestedInteger nestedInteger: nestedList) {
if (nestedInteger.isInteger()) {
} else {
flattenList(nestedInteger.getList());
}
}
}

@Override
public Integer next() {
if (!hasNext()) {
throw new NoSuchElementException();
}
return integers.get(position++);
}

@Override
public boolean hasNext() {
return position < integers.size();
}
}

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

### Approach #2 Stack

``````import java.util.NoSuchElementException;

public class NestedIterator implements Iterator<Integer> {

// In Java, the Stack class is considered deprecated. Best practice is to use
// a Deque instead. We'll use addFirst() for push, and removeFirst() for pop.
private Deque<NestedInteger> stack;

public NestedIterator(List<NestedInteger> nestedList) {
// The constructor puts them on in the order we require. No need to reverse.
stack = new ArrayDeque(nestedList);
}

@Override
public Integer next() {
// As per java specs, throw an exception if there's no elements left.
if (!hasNext()) throw new NoSuchElementException();
// hasNext ensures the stack top is now an integer. Pop and return
// this integer.
return stack.removeFirst().getInteger();
}

@Override
public boolean hasNext() {
// Check if there are integers left by getting one onto the top of stack.
makeStackTopAnInteger();
// If there are any integers remaining, one will be on the top of the stack,
// and therefore the stack can't possibly be empty.
return !stack.isEmpty();
}

private void makeStackTopAnInteger() {
// While there are items remaining on the stack and the front of
// stack is a list (i.e. not integer), keep unpacking.
while (!stack.isEmpty() && !stack.peekFirst().isInteger()) {
// Put the NestedIntegers onto the stack in reverse order.
List<NestedInteger> nestedList = stack.removeFirst().getList();
for (int i = nestedList.size() - 1; i >= 0; i--) {
}
}
}
}``````

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