leetcode解题: Flatten Nested List Iterator (341)

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].

解法1： Stack， O(N) Time, O(N) Space, N is the number of total items

这题是用iteration的办法解决递归的问题，一般此类问题容易想到用stack解决。这题哪里可以用stack呢？每一个nestedInteger都可能是一个list of nestedInteger， 那么我们对于每一个元素，如果是单数，则任务完成，如果是一个list， 那么我们把所有的元素都推入栈中，直到第一个元素是单数或者到遍历结束为止。 由于栈是FILO, 所以每次推入的时候需要从后往前推。

/**
 * // This is the interface that allows for creating nested lists.
 * // You should not implement it, or speculate about its implementation
 * class NestedInteger {
 *   public:
 *     // Return true if this NestedInteger holds a single integer, rather than a nested list.
 *     bool isInteger() const;
 *
 *     // Return the single integer that this NestedInteger holds, if it holds a single integer
 *     // The result is undefined if this NestedInteger holds a nested list
 *     int getInteger() const;
 *
 *     // Return the nested list that this NestedInteger holds, if it holds a nested list
 *     // The result is undefined if this NestedInteger holds a single integer
 *     const vector<NestedInteger> &getList() const;
 * };
 */
class NestedIterator {
public:
    NestedIterator(vector<NestedInteger> &nestedList) {
        for (int i = nestedList.size() - 1; i >= 0; --i) {
            s.push(nestedList[i]);
        }
    }

    int next() {
        int res = s.top().getInteger();
        s.pop();
        return res;
    }

    bool hasNext() {
        while (!s.empty()) {
            if (s.top().isInteger()) {
                return true;
            }
            vector<NestedInteger> cur = s.top().getList();
            s.pop();
            for (int i = cur.size() - 1; i >= 0; --i) {
                s.push(cur[i]);
            }
        }
        return false;
    }
private:
    stack<NestedInteger> s;
};

/**
 * Your NestedIterator object will be instantiated and called as such:
 * NestedIterator i(nestedList);
 * while (i.hasNext()) cout << i.next();
 */

/**
 * // 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;
    public NestedIterator(List<NestedInteger> nestedList) {
        stack = new Stack<NestedInteger>();
        for (int i = nestedList.size() - 1; i >= 0; i--) {
            stack.push(nestedList.get(i));
        }
    }

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

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