Monotonic Stack #21
Description
单调栈框架
以单调递增栈为例：
Next Less Element
Previous Less Element
for (int i = 0; i < n; i++) {
right[i] = n-i; // 某些元素不能进到pop循环，所以赋初值
while (!stack.empty() && A[stack.peek()] > A[i]) {
int peek = stack.pop();
right[peek] = i - peek; // pop时操作 Next Less Element
}
// 栈为空时需要特殊赋值
left[i] = (stack.empty() ? i+1 : i - stack.peek()); // push时操作，Previous Less Element
stack.push(i);
}