341. Flatten Nested List Iterator

341. Flatten Nested List Iterator

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].

• Stack

  public class NestedIterator implements Iterator<Integer> {
    Stack<NestedInteger> stack;
    public NestedIterator(List<NestedInteger> nestedList) {
        stack = new Stack<>();
        for(int i = nestedList.size()-1; i >= 0; i--) {
            stack.push(nestedList.get(i));
        }
    }
    public Integer next() {
        return stack.pop().getInteger();
    }
    public boolean hasNext() {
        if(stack.isEmpty()) return false;
        if(!stack.peek().isInteger()) {
            List<NestedInteger> nestedList = stack.pop().getList();
            for(int i = nestedList.size()-1; i >= 0; i--) {
                stack.push(nestedList.get(i));
            }
            hasNext(); // recursive flatten lists
        }
        return !stack.isEmpty(); // may flatten empty nested lists
    }
  }