[Leetcode] 341. Flatten Nested List Iterator 解题报告

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本文介绍了一种使用栈来实现嵌套列表迭代器的方法，该方法能够将嵌套的整数列表摊平为一维序列。通过逆序压栈和检查栈顶元素的方式，确保每次调用 next() 函数都能返回下一个整数值。

题目：

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.

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].

思路：

我们用栈来解决这个问题：首先将NestedInteger数组里面的元素逆序加入栈中，然后检查栈顶元素，如果是list，则将其弹出，并将其内部元素逆序加入栈中，重复这一过程直到栈顶元素是Integer。在next()函数中，首先弹出栈顶元素并记录其Integer的值。但是此时还要检查栈顶元素是不是Integer，如果不是，还需要采用和上面相同的方法，摊平栈顶元素直到其顶部元素是Integer而不是list，最后再返回刚才记录的Integer的值。

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) {
        for (auto it = nestedList.rbegin(); it != nestedList.rend(); ++it) {
            st.push(*it);
        }
        while (!st.empty() && !st.top().isInteger()) {
            NestedInteger ni = st.top();
            st.pop();
            vector<NestedInteger> &list = ni.getList();
            for (auto it = list.rbegin(); it != list.rend(); ++it) {
                st.push(*it);
            }
        }
    }

    int next() {
        int ret = st.top().getInteger();
        st.pop();
        while (!st.empty() && !st.top().isInteger()) {
            NestedInteger ni = st.top();
            st.pop();
            vector<NestedInteger> &list = ni.getList();
            for (auto it = list.rbegin(); it != list.rend(); ++it) {
                st.push(*it);
            }
        }
        return ret;
    }

    bool hasNext() {
        return !st.empty();
    }
private:
    stack<NestedInteger> st;
};

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