Problem 341: Flatten Nested List Iterator

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

思路

• 用一个 stack 来维护元素的进出。

• 元素进栈的时候，从后往前压入栈，这样取出的时候正好是从前到后。

• 对于 hasNext()，如果栈内元素是 Integer 的话，那毫无疑问是 true；如果是 list 的话，可以像之前一样，从后往前把子 list 内的元素压入栈中查看，

/**
 * // 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> {

    Stack<NestedInteger> stack = new Stack<NestedInteger>();
    public NestedIterator(List<NestedInteger> nestedList) {
        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 curr = stack.peek();
            if (curr.isInteger()) {
                return true;
            }

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