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.

Notice

You don't need to implement the remove method.

Example

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].

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

1. 用Stack实现iterator
2. 需要建一个中间栈temp stack，倒插实现pop的顺序
3. 准备stack的主功能放在hasNext中

/**
 * // 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();
 * }
 */
import java.util.Iterator;

public class NestedIterator implements Iterator<Integer> {

    Stack<NestedInteger> stack;

    private void PushListToStack(List<NestedInteger> list) {
        Stack<NestedInteger> temp = new Stack<NestedInteger>();
        for (NestedInteger item : list) {
            temp.push(item);
        }

        while (!temp.isEmpty()) {
            stack.push(temp.pop());
        }
    }

    public NestedIterator(List<NestedInteger> nestedList) {
        // Initialize your data structure here.
        stack = new Stack<NestedInteger>();
        PushListToStack(nestedList);
    }

    // @return {int} the next element in the iteration
    @Override
    public Integer next() {
        // Write your code here
        if (hasNext()) {
            return stack.pop().getInteger();
        } else {
            return null;
        }
    }

    // @return {boolean} true if the iteration has more element or false
    @Override
    public boolean hasNext() {
        // Write your code here
        while (!stack.isEmpty() && !stack.peek().isInteger()) {
            PushListToStack(stack.pop().getList());
        }

        return !stack.isEmpty();
    }

    @Override
    public void remove() {}
}

/**
 * Your NestedIterator object will be instantiated and called as such:
 * NestedIterator i = new NestedIterator(nestedList);
 * while (i.hasNext()) v.add(i.next());
 */