【leetcode】341. Flatten Nested List Iterator

題目：
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.

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思路：
抄作業https://leetcode.com/problems/flatten-nested-list-iterator/discuss/80146/Real-iterator-in-Python-Java-C%2B%2B，思路是用stack保存vector<NestedInteger>的迭代器，如果迭代器到達end，則將其彈出。

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代碼實現：

/**
 * // 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:
    stack<vector<NestedInteger>::iterator> begins, ends;
    
    NestedIterator(vector<NestedInteger> &nestedList) {
        begins.push(nestedList.begin());
        ends.push(nestedList.end());
    }
    
    int next() {
        hasNext();
        return (begins.top()++)->getInteger();
    }
    
    bool hasNext() {
        while (begins.size()){
            if (begins.top() == ends.top()){
                begins.pop();
                ends.pop();
            }else {
                auto x = begins.top();
                if (x->isInteger()){
                    return true;
                }
                begins.top()++;
                begins.push(x->getList().begin());
                ends.push(x->getList().end());
            }
        }
        
        return false;
    }
};

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

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參考：
https://leetcode.com/problems/flatten-nested-list-iterator/discuss/80146/Real-iterator-in-Python-Java-C%2B%2B