Description

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:
Given the list [[1,1],2,[1,1]],

By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,1,2,1,1].

Example 2:
Given the list [1,[4,[6]]],

By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,4,6].

Solution

Stack, hasNext time O(n), next time O(1), space O(n)

用stack即可实现。原本写的solution是在next中处理stack内元素，hasNext只判断stack是否为空，但这会有问题。

考虑test case: [[]

压栈之后，栈不为空，hasNext返回true，但实际上next并没有值。这说明我们需要在hasNext中处理栈内元素，才能handle这种情况。

/**
 * // 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 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));
        }
    }

    @Override
    public Integer next() {
        return stack.pop().getInteger();
    }

    @Override
    public boolean hasNext() {
        while (!stack.empty()) {
            if (stack.peek().isInteger()) {
                return true;
            }
            
            NestedInteger nested = stack.pop();

            for (int i = nested.getList().size() - 1; i >= 0; --i) {
                stack.push(nested.getList().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();
 */