[leetcode] 341. Flatten Nested List Iterator

最新推荐文章于 2025-09-06 17:22:57 发布

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最新推荐文章于 2025-09-06 17:22:57 发布

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本文介绍两种实现嵌套整数列表迭代器的方法。一种利用栈进行迭代读取元素；另一种通过递归提取所有整数到队列中，再利用队列特性实现迭代功能。

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].

解法一：

主要思想是借用一个stack，然后用迭代的思想把单个integer读出来。

/**
 * // 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() {
        NestedInteger t = s.top(); s.pop();
        return t.getInteger();
        
    }

    bool hasNext() {
        while(!s.empty()){
            NestedInteger t = s.top();
            if(t.isInteger()) return true;
            s.pop();
            for(int i = t.getList().size()-1; i>=0; --i)
                s.push(t.getList()[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();
 */

解法二：

先recursive的提取出nestlist中的integer，放到queue中，然后使用queue的函数去实现hasnext和next。

/**
 * // 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) {
        makeQueue(nestedList);
    }

    int next() {
        int res = q.front();
        q.pop();
        return res;
        
    }

    bool hasNext() {
        return !q.empty();
    }
private:
    queue<int> q;
    void makeQueue(vector<NestedInteger> &nestedList){
        for(auto a: nestedList){
            if(a.isInteger()) q.push(a.getInteger());
            else makeQueue(a.getList());
        }
    }
    
};

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