STL: 从reverse到iterator

leetcode:Next Permutation

class Solution {
public:
    void nextPermutation(vector<int> &num) {
        auto rfirst = num.rbegin();
        auto rlast = num.rend();
        auto pivot = next(rfirst);
        while(pivot!=rlast&&*pivot>=*prev(pivot)) {
            ++pivot;
        }
        if(pivot==rlast) {
            reverse(rfirst,rlast);
            return;
        }
        auto change = find_if(rfirst,pivot,bind1st(less<int>(),*pivot));
        swap(*change,*pivot);
        reverse(rfirst,pivot);
    }
};

首先看一下reverse函数的说明：

template <class BidirectionalIterator>
  void reverse (BidirectionalIterator first, BidirectionalIterator last);

这个BidirectionalIterator和它的字面意思一样，就是iterators that can be used to access the sequence of elements in a range in both directions (towards the end and towards the beginning).

然后再看一下rfirst和rlast都是什么迭代器：

num.rbegin()返回的是Returns a reverse iterator pointing to the last element in the vector ，num.rend()返回的是 a reverse iterator pointing to the theoretical element preceding the first element in the vector (which is considered its reverse end).  这两个迭代器都是reverse_iterator。这个reverse_iterator事实上都是 random access iterator 类型。

之前经常用的begin()和end()，它们返回的也是random access iterator。

而All random-access iterators are also valid bidirectional iterators.

所以就都可以当成Reverse的参数，来完成vector在指定范围内的反转了。