链接：LintCode 炼码 - ChatGPT！更高效的学习体验！

题解：

1.从左往右维护一个单调递减，栈的长度就是，可以看到最多的高楼

2.从右往后也是维护一个单调递减的栈

class Solution {
public:
    /**
     * @param arr: the height of all buildings
     * @return: how many buildings can he see at the location of each building
     */
    vector<int> tallBuilding(vector<int> &arr) {
        // Write your code here.

        if (arr.size() <= 0) {
            return arr;
        }
        std::vector<int> result(arr.size(), 1);
        calc(arr, result, 0, arr.size(), 1);
        calc(arr, result, arr.size()-1, -1, -1);
        return result;
    }
private:
    void calc(const std::vector<int>& arr, std::vector<int>& result, int begin, int end, int step) {
        std::stack<int> sta;
        for (int i = begin; i != end; i += step) {
            result[i] += sta.size();
            while (!sta.empty() && arr[sta.top()] <= arr[i]) {
                sta.pop();
            }
            sta.push(i);
        }
    }
};