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

Examples

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].

Solutions

// 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();
}

/**
 * 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();
 */

Follow up

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