单调栈：

保持栈内的元素始终递增或递减。

java">单调递增 待处理数组{1,5,2,5,7,2,8}
public void sameyIncrease(int[] nums) {Stack<Integer> stack = new Stack<>();for(int i = 0; i < nums.length; i++) {//当栈空的时候可以直接进栈或者要进栈的数大于等于栈顶的数if(stack.isEmpty() || nums[stack.peek()] <= nums[i]) {stack.push(i);} else {//当进栈的数不满足规律的时候我们将原本栈内的数弹出，直至满足规律或栈空了while(!stack.isEmpty() && nums[stack.peek()] >= nums[i]) {stack.pop();}stack.push(i);}}
}

java">单调递减 待处理数组{1,5,2,5,7,2,8}
public void sameyIncrease(int[] nums) {Stack<Integer> stack = new Stack<>();for(int i = 0; i < nums.length; i++) {if(stack.isEmpty() || nums[stack.peek()] >= nums[i]) {stack.push(i);} else {while(!stack.isEmpty() && nums[stack.peek()] >= nums[i]) {stack.pop();}stack.push(i);}}
}