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].
Solution:
/**
* // 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> {
Deque<Iterator<NestedInteger>> stack = new ArrayDeque<>();
nextInt = null;
public NestedIterator(List<NestedInteger> nestedList) {
Iterator<NestedInteger> it = nestedList.iterator();
stack.push(it);
}
@Override
public Integer next() {
Integer result = nextInt.getInteger();
nextInt = null;
return result;
}
@Override
public boolean hasNext() {
// keep checking the iterator and stack, until find a interger
if(nextInt!=null) return true;
while(!stack.isEmpty()) {
if(stack.peek().hasNext()) {
nextInt = stack.peek().next();
if(nextInt.isInteger()) return true; // get integer, return
else stack.push(nextInt.getList().iterator());// get list, push into stack and search next.
}else stack.pop();
}
return false;
}
}
/**
* Your NestedIterator object will be instantiated and called as such:
* NestedIterator i = new NestedIterator(nestedList);
* while (i.hasNext()) v[f()] = i.next();
*/
