341. Flatten Nested List Iterator

341. Flatten Nested List Iterator

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.

Example 1:

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].

Example 2:

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

Constraints:

• 1 <= nestedList.length <= 500

• The values of the integers in the nested list is in the range [-106, 106].

My Solutions:

方法1：因为要保持element的order，在constructor里从后往前遍历元素，加到stack里面，这样保证排在后面的element最后出。在hasNext里处理元素，如果最上面的元素不是integer，把他们从后往前加入到stack里。

/**
 * // 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();
 * }
 */
public class NestedIterator implements Iterator<Integer> {
    
    Stack<NestedInteger> stack = new Stack<>();

    public NestedIterator(List<NestedInteger> nestedList) {
        if (nestedList == null) return;
        for (int i = nestedList.size() - 1; i >= 0; i--)
            // 把NestedInteger push到stack里
            stack.push(nestedList.get(i));
    }

    @Override
    public Integer next() {
        // 注意hasNext一定先被call，hasNext已经处理过了NestedInteger
        return stack.pop().getInteger();
    }

    @Override
    public boolean hasNext() {
        while (!stack.isEmpty()) {
            NestedInteger top = stack.peek();
            if (top.isInteger()) return true;
            else {
                stack.pop();
                for (int i = top.getList().size() - 1; i >= 0; i--) {
                    stack.push(top.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();
 */

方法2：写一个function先把所有元素都先加进一个list，用来存贮integer。用一个int指针代表已经next出到第几个元素。

public class NestedIterator implements Iterator<Integer> {

    List<Integer> list;
    int index;

    public NestedIterator(List<NestedInteger> nestedList) {
        if (nestedList == null) return;
        this.list = new ArrayList<>();
        this.index = 0;
        for (NestedInteger n : nestedList) addIntegerToList(n);
    }

    public void addIntegerToList(NestedInteger n) {
        if (n.isInteger()) this.list.add(n.getInteger());
        else {
            List<NestedInteger> tmp = n.getList();
            for (NestedInteger tmpN : tmp) addIntegerToList(tmpN);
        }
    }

    @Override
    public Integer next() {
        return this.list.get(index++);
    }

    @Override
    public boolean hasNext() {
        if (index < this.list.size()) return true;
        else return false;
    }
}