341. Flatten Nested List Iterator

问题描述：

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].

解题思路：

这道题我们可以用栈来辅助解答

设置一个存储NestedInteger的栈并将list里的对象全部压入栈中

在hasNext方法中，我们需要判断的是，当前是否存在有效的对象。

当前栈顶有三种可能：

　　1.栈为空，显然这时hasNext返回false

　　2.栈顶为对象：此时可以返回true

　　3.栈顶为list：此时我们要把list拆解并重新入栈，直至栈顶为一个对象

需要注意的是！

若栈顶为list嵌套着空的list如：[ []， [] ]

看起来像是一个list里嵌套了两个list，我们要对其进行拆解。

代码：

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

    int next() {
        NestedInteger list = stk.top();
        int ret = list.getInteger();
        stk.pop();
        return ret;
    }

    bool hasNext() {
        while(!stk.empty()){
            NestedInteger temp = stk.top();
            if(!temp.isInteger()){
                stk.pop();
                vector<NestedInteger> list = temp.getList();
                if(!list.empty()){
                    pushToStack(list);
                }
            }else{
                return true;
            }
        }
        
        return false;
    }
private:
    stack<NestedInteger> stk;
    void pushToStack(vector<NestedInteger> &list){
        int n = list.size();
        for(int i = n-1; i > -1; i--){
            stk.push(list[i]);
        }
    }
};

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