341 Flatten Nested List Iterator

341. Flatten Nested List Iterator

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.

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].

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分析&代码

需要注意的是，有可能出现输入是[[]]的情况，所以在hasNext()的判断中，需要每次展开之后都检查stack是不是为空。

代码把stack误写成queue了，但是懒得改了

/**
 * // 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<NestedInteger> queue;

    public NestedIterator(List<NestedInteger> nestedList) {
        queue = new ArrayDeque<>();
        for(int i = nestedList.size() - 1; i >= 0; i--) {
            queue.offerFirst(nestedList.get(i));
        }
    }

    @Override
    public Integer next() {
        return queue.pollFirst().getInteger();
    }

    @Override
    public boolean hasNext() {
        while(!queue.isEmpty() && !queue.peekFirst().isInteger()) {
            List<NestedInteger> nextList = queue.pollFirst().getList();
            for(int i = nextList.size() - 1; i >= 0; i--) {
                queue.offerFirst(nextList.get(i));
            }
        }
        return !queue.isEmpty();
    }
}

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