# 341.Flatten-Nested-List-Iterator

## 341. Flatten Nested List Iterator

## 题目地址

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

## 题目描述

```
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].
```

## 代码

### Approach #1 Recursion

```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 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()) {
        integers.add(nestedInteger.getInteger());
      } 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

```java
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--) {
                stack.addFirst(nestedList.get(i));
            }
        }
    }
}
```


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