C++ 单调栈

C++单调栈模版代码：

#include<iostream>
#include<vector>
#include<stack>
using namespace std;/*单调栈模版*/
#define maxn 100001
#define inf 2000000000template<typename T>
bool cmp(T a, T b) {return a > b;
}
// ans[i]代表从i往左走，找到的第一个满足条件的下标
template<typename T>
void findFirstMeetOnleft(int n, T h[], int ans[]) {stack<int> stk;h[0] = inf;stk.push(0);for (int i = 1; i <= n; ++i) {while(!cmp(h[stk.top()],h[i])) {stk.pop();}ans[i] = stk.top();stk.push(i);}
}template<typename T>
void reverseArray(int n, T arr[]) {for (int i = 1; i <= n / 2; ++i) {T tmp = arr[i];arr[i] = arr[n + 1 - i];arr[n + 1 - i] = tmp;}
}/*单调栈模版*/
int h[maxn], ans[maxn];int main() {int n;cin >> n;for (int i = 1; i <= n; ++i) {cin >> h[i];}findFirstMeetOnleft(n, h, ans);for (int i = 1; i <= n; ++i) {cout << ans[i] << " ";}cout << endl;return 0;
}

代码练习 1，伦太郎的等待，对应蓝桥云课 代码见下

#include<iostream>
#include<vector>
#include<stack>
using namespace std;/*单调栈模版*/
#define maxn 100001
#define inf 2000000000template<typename T>
bool cmp(T a, T b) {return a >= b;
}
// ans[i]代表从i往左走，找到的第一个满足条件的下标
template<typename T>
void findFirstMeetOnleft(int n, T h[], int ans[]) {stack<int> stk;h[0] = inf;stk.push(0);for (int i = 1; i <= n; ++i) {while(!cmp(h[stk.top()],h[i])) {stk.pop();}ans[i] = stk.top();stk.push(i);}
}template<typename T>
void reverseArray(int n, T arr[]) {for (int i = 1; i <= n / 2; ++i) {T tmp = arr[i];arr[i] = arr[n + 1 - i];arr[n + 1 - i] = tmp;}
}/*单调栈模版*/
int h[maxn], ans[maxn];int main() {int n;cin >> n;for (int i = 1; i <= n; ++i) {cin >> h[i];}findFirstMeetOnleft(n, h, ans);int ret = 0;for (int i = 1; i <= n; ++i) {ret += i - ans[i] - 1;}cout << ret << endl;return 0;
}

代码练习2 对应蓝桥云课 百亿富翁 代码见下

#include<iostream>
#include<vector>
#include<stack>
using namespace std;/*单调栈模版*/
#define maxn 700001
#define inf 2000000000template<typename T>
bool cmp(T a, T b) {return a > b;
}
// ans[i]代表从i往左走，找到的第一个满足条件的下标
template<typename T>
void findFirstMeetOnleft(int n, T h[], int ans[]) {stack<int> stk;h[0] = inf;stk.push(0);for (int i = 1; i <= n; ++i) {while(!cmp(h[stk.top()],h[i])) {stk.pop();}ans[i] = stk.top();stk.push(i);}
}template<typename T>
void reverseArray(int n, T arr[]) {for (int i = 1; i <= n / 2; ++i) {T tmp = arr[i];arr[i] = arr[n + 1 - i];arr[n + 1 - i] = tmp;}
}/*单调栈模版*/
int h[maxn], ans[maxn];int main() {int n;cin >> n;for (int i = 1; i <= n; ++i) {cin >> h[i];}findFirstMeetOnleft(n, h, ans);for (int i = 1; i <= n; ++i) {if (ans[i] == 0) ans[i] = -1;cout << ans[i] << ' ';}cout << endl;reverseArray(n, h);findFirstMeetOnleft(n, h, ans);reverseArray(n, ans);reverseArray(n, h);for (int i = 1; i <= n; ++i) {if (ans[i] == 0) ans[i] = -1;else ans[i] = (n + 1) - ans[i];cout << ans[i] << ' ';}cout << endl;return 0;
}

代码练习3 对应蓝桥云课 最大区间 代码见下

#include<iostream>
#include<vector>
#include<stack>
using namespace std;/*单调栈模版*/
#define maxn 300001
#define inf -1template<typename T>
bool cmp(T a, T b) {return a < b;
}
// ans[i]代表从i往左走，找到的第一个满足条件的下标
template<typename T>
void findFirstMeetOnleft(int n, T h[], int ans[]) {stack<int> stk;h[0] = inf;stk.push(0);for (int i = 1; i <= n; ++i) {while(!cmp(h[stk.top()],h[i])) {stk.pop();}ans[i] = stk.top();stk.push(i);}
}template<typename T>
void reverseArray(int n, T arr[]) {for (int i = 1; i <= n / 2; ++i) {T tmp = arr[i];arr[i] = arr[n + 1 - i];arr[n + 1 - i] = tmp;}
}/*单调栈模版*/
int h[maxn], l[maxn], r[maxn];int main() {int n;cin >> n;for (int i = 1; i <= n; ++i) {cin >> h[i];}findFirstMeetOnleft(n, h, l);reverseArray(n, h);findFirstMeetOnleft(n, h, r);reverseArray(n, h);reverseArray(n, r);for (int i = 1; i <= n; ++i) {r[i] = (n + 1) - r[i];}long long max = 0;for (int i = 1; i <= n; ++i) {long long x = (r[i] - 1) - (l[i] + 1) + 1;x = x * h[i];if (x > max) {max = x;}}cout << max << endl;return 0;
}