[leetcode] 341. Flatten Nested List Iterator

Posted 2020-07-28

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].

 

Solution:

class NestedIterator {
public:
    NestedIterator(vector<NestedInteger> &nestedList) 
    {
        for (int i = nestedList.size() - 1; i >= 0; i--)
        {
            stk.push(nestedList[i]);
        }
    }

    int next() 
    {
         NestedInteger t = stk.top(); // access top value
         stk.pop();
         return t.getInteger();
    }

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