503. 下一个更大元素 II

class Solution {
    public int[] nextGreaterElements(int[] nums) {
        if(nums == null || nums.length <= 1){
            return new int[]{-1};
        }
        Stack<Integer> monoStack = new Stack<>();
        int[] res = new int[nums.length];
        Arrays.fill(res, -1);

        for(int i = 0; i < nums.length * 2; i++){
            while(!monoStack.isEmpty() && nums[monoStack.peek()] < nums[i % nums.length]){
                res[monoStack.pop()] = nums[i % nums.length];
            }
            monoStack.push(i % nums.length);
        }

        return res;
    }
}

42. 接雨水

双指针法：

class Solution {
    public int trap(int[] height) {
        int n = height.length;
        int[] leftMax = new int[n];
        int[] rightMax = new int[n];

        int leftHighest = 0;
        for(int i = 0; i < n; i++){
            leftMax[i] = leftHighest;
            leftHighest = Math.max(leftHighest, height[i]);
        }

        int rightHeighest = 0;
        for(int i = n - 1; i >= 0; i--){
            rightMax[i] = rightHeighest;
            rightHeighest = Math.max(rightHeighest, height[i]);
        }

        int res = 0;
        for(int i = 0; i < n; i++){
            int minHeight = Math.min(leftMax[i], rightMax[i]);
            if(minHeight > height[i]){
                res += minHeight - height[i];
            }
        }

        return res;
    }
}

单调栈法：

class Solution {
    public int trap(int[] height) {
        Stack<Integer> monoStack = new Stack<>();
        int res = 0;
        for(int i = 0; i < height.length; i++){
            while(!monoStack.isEmpty() && height[monoStack.peek()] < height[i]){
                int mid = monoStack.pop();
                if(!monoStack.isEmpty()){
                    int h = Math.min(height[monoStack.peek()], height[i]) - height[mid];
                    int w = i - monoStack.peek() - 1;
                    res += h * w;
                }
            }
            monoStack.push(i);
        }
        return res;
    }
}