42. 接雨水 84.柱状图中最大的矩形

42. 接雨水

单调栈法：

class Solution {
    public int trap(int[] height) {
        int size = height.length;
        if (size <= 2) return 0;
        Stack<Integer> stack = new Stack<Integer>();
        stack.push(0);
        int sum = 0;
        for(int i = 1;i < size;i++){
            if(height[stack.peek()] >= height[i]){
                stack.push(i);
            }else if(height[stack.peek()] == height[i]){
                stack.pop();
                stack.push(i);
            }else{
                while(!stack.isEmpty() && height[stack.peek()] < height[i]){
                    int mid = stack.peek();
                    stack.pop();
                    if(!stack.isEmpty()){
                        int left = stack.peek();
                        int right = i;
                        int w = right - left - 1;
                        int h = Math.min(height[left] - height[mid],height[right] - height[mid]);
                        sum += w * h;
                    }
                }
                stack.push(i);
            }
        }
        return sum;
    }
}

最优解：
双指针法

class Solution {
    public int trap(int[] height) {
        int left = 0;
        int right = height.length - 1;
        int l_max = 0;
        int r_max = 0;
        int result = 0;
        while(left <= right){
            l_max = Math.max(l_max,height[left]);
            r_max = Math.max(r_max,height[right]);
            if(l_max < r_max){
                result += l_max - height[left];
                left++;
            }else{
                result += r_max - height[right];
                right--;
            }
        }
        return result;
    }
}

84.柱状图中最大的矩形

单调栈法：

class Solution {
    public int largestRectangleArea(int[] heights) {
        int[] newHeights = new int[heights.length + 2];
        newHeights[0] = 0;
        newHeights[newHeights.length-1] = 0;
        for(int i = 1;i < newHeights.length - 1;i++){
            newHeights[i] = heights[i-1];
        }
        Stack<Integer> stack = new Stack<>();
        stack.push(0);
        int result = 0;
        for(int i = 1;i < newHeights.length;i++){
            if(newHeights[stack.peek()] < newHeights[i]){
                stack.push(i);
            }else if(newHeights[stack.peek()] == newHeights[i]){
                stack.pop();
                stack.push(i);
            }else{
                while(!stack.isEmpty() && newHeights[stack.peek()] > newHeights[i]){
                    int mid = stack.peek();
                    stack.pop();
                    if(!stack.isEmpty()){
                        int left = stack.peek();
                        int right = i;
                        int w = i - left - 1;
                        int h = newHeights[mid];
                        result = Math.max(result,w * h);
                    }
                }
                stack.push(i);
            }
        }
        return result;
    }
}