代码随想录算法训练营第49天

42. 接雨水

接雨水这道题目是面试中特别高频的一道题，也是单调栈应用的题目，大家好好做做。

建议是掌握双指针和单调栈，因为在面试中写出单调栈可能有点难度，但双指针思路更直接一些。

在时间紧张的情况有，能写出双指针法也是不错的，然后可以和面试官在慢慢讨论如何优化。

java 复制代码

class Solution {
    public int trap(int[] height) {
        int total = 0;
        for (int index = 0; index < height.length; index++) {
            if (index == 0 || index == height.length - 1) continue;

            int rightMax = height[index]; 
            int leftMax = height[index]; 
            for (int right = index + 1; right < height.length; right++) {
                rightMax = Math.max(rightMax, height[right]);
            }
            for (int left = index - 1; left >= 0; left--) {
                leftMax = Math.max(leftMax, height[left]);
            }
            int waterHeight = Math.min(leftMax, rightMax) - height[index];
            if (waterHeight > 0) total += waterHeight;
        }
        return total;
    }
}

柱状图中最大的矩形

有了之前单调栈的铺垫，这道题目就不难了。

java 复制代码

class Solution {
    public int largestRectangleArea(int[] heights) {
        int[] extendedHeights = new int[heights.length + 2];
        System.arraycopy(heights, 0, extendedHeights, 1, heights.length);
        extendedHeights[0] = 0;
        extendedHeights[heights.length + 1] = 0;

        Stack<Integer> stack = new Stack<>();
        stack.push(0);

        int maxArea = 0;
        for (int i = 1; i < extendedHeights.length; i++) {
            while (!stack.isEmpty() && extendedHeights[i] < extendedHeights[stack.peek()]) {
                int topIndex = stack.pop();
                int width = i - stack.isEmpty() ? 0 : i - stack.peek() - 1;
                int height = extendedHeights[topIndex];
                maxArea = Math.max(maxArea, width * height);
            }
            stack.push(i);
        }
        return maxArea;
    }
}

代码随想录