代码随想录算法训练营第五十九天|503.下一个更大元素II、42. 接雨水

503.下一个更大元素II

• 刷题https://leetcode.cn/problems/next-greater-element-ii/description/
• 文章讲解https://programmercarl.com/0503.%E4%B8%8B%E4%B8%80%E4%B8%AA%E6%9B%B4%E5%A4%A7%E5%85%83%E7%B4%A0II.html
• 视频讲解https://www.bilibili.com/video/BV15y4y1o7Dw/?vd_source=af4853e80f89e28094a5fe1e220d9062

• 题解：

class Solution {
    public int[] nextGreaterElements(int[] nums) {
        int n = nums.length;
        int[] ret = new int[n];
        Arrays.fill(ret, -1);
        Deque<Integer> stack = new LinkedList<Integer>();
        for (int i = 0; i < n * 2 - 1; i++) {
            while (!stack.isEmpty() && nums[stack.peek()] < nums[i % n]) {
                ret[stack.pop()] = nums[i % n];
            }
            stack.push(i % n);
        }
        return ret;
    }
}

42. 接雨水

• 刷题https://leetcode.cn/problems/trapping-rain-water/description/
• 文章讲解https://programmercarl.com/0042.%E6%8E%A5%E9%9B%A8%E6%B0%B4.html
• 视频讲解https://www.bilibili.com/video/BV1uD4y1u75P/?vd_source=af4853e80f89e28094a5fe1e220d9062

• 题解：

// 单调栈法
class Solution {
    public int trap(int[] height){
        int size = height.length;

        if (size <= 2) return 0;

        // in the stack, we push the index of array
        // using height[] to access the real height
        Stack<Integer> stack = new Stack<Integer>();
        stack.push(0);

        int sum = 0;
        for (int index = 1; index < size; index++){
            int stackTop = stack.peek();
            if (height[index] < height[stackTop]){
                stack.push(index);
            }else if (height[index] == height[stackTop]){
                // 因为相等的相邻墙，左边一个是不可能存放雨水的，所以pop左边的index, push当前的index
                stack.pop();
                stack.push(index);
            }else{
                //pop up all lower value
                int heightAtIdx = height[index];
                while (!stack.isEmpty() && (heightAtIdx > height[stackTop])){
                    int mid = stack.pop();

                    if (!stack.isEmpty()){
                        int left = stack.peek();

                        int h = Math.min(height[left], height[index]) - height[mid];
                        int w = index - left - 1;
                        int hold = h * w;
                        if (hold > 0) sum += hold;
                        stackTop = stack.peek();
                    }
                }
                stack.push(index);
            }
        }

        return sum;
    }
}