单调栈

每日温度

leetcode题目链接

给定一个整数数组 temperatures ，表示每天的温度，返回一个数组 answer ，其中 answer[i] 是指对于第 i 天，下一个更高温度出现在几天后。如果气温在这之后都不会升高，请在该位置用 0 来代替。

思路草稿

每日温度

题解

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class Solution {
    public int[] dailyTemperatures(int[] temperatures) {
        int[] res = new int[temperatures.length];
        Stack<Integer> stack = new Stack<>();
        stack.push(0);

        for(int i = 1;i<temperatures.length;i++){
            if(temperatures[i]<=temperatures[stack.peek()]){
                stack.push(i);
            } else {
                while(!stack.isEmpty()&&temperatures[i]>temperatures[stack.peek()]){
                    res[stack.peek()] = i - stack.peek(); 
                    stack.pop();
                }
                stack.push(i);
            }
        }
        return res;
    }
}

接雨水

leetcode题目链接

给定 n 个非负整数表示每个宽度为 1 的柱子的高度图，计算按此排列的柱子，下雨之后能接多少雨水。

思路草稿

接雨水

题解

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class Solution {
    public int trap(int[] height) {
        if(height==null||height.length==0){
            return 0;
        }

        int h = 0;
        int w = 0;
        int res = 0;

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

        for(int i = 1;i<height.length;i++){
            if(height[i]<height[stack.peek()]){
                stack.push(i);
            } else if(height[i]==height[stack.peek()]){
                stack.pop();
                stack.push(i);
            } else {
                while(!stack.isEmpty()&&height[i]>height[stack.peek()]){
                    int mid = stack.pop();
                    if(!stack.isEmpty()){
                        h = Math.min(height[stack.peek()],height[i])-height[mid];
                        w = i-stack.peek()-1;
                        res += h*w;
                    }
                }
                stack.push(i);
            }
        }

        return res;
    }
}

柱状图中最大的矩形

leetcode题目链接

给定 n 个非负整数，用来表示柱状图中各个柱子的高度。每个柱子彼此相邻，且宽度为 1 。

求在该柱状图中，能够勾勒出来的矩形的最大面积。

思路草稿

柱状图中最大的矩形

题解

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class Solution {
    public int largestRectangleArea(int[] heights) {
        if(heights==null||heights.length==0){
            return 0;
        }
        int[] newHeights = new int[heights.length+2];
        newHeights[0] = 0;
        newHeights[newHeights.length-1] = 0;
        for(int i = 0;i<heights.length;i++){
            newHeights[i+1] = heights[i];
        }

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

        int res = 0;

        for(int i = 1;i<newHeights.length;i++){
            if(newHeights[i]>newHeights[stack.peek()]){
                stack.push(i);
            } else if(newHeights[i]==newHeights[stack.peek()]){
                stack.pop();
                stack.push(i);
            } else {
                while(!stack.isEmpty()&&newHeights[i]<newHeights[stack.peek()]){
                    int mid = stack.pop();
                    if(!stack.isEmpty()){
                        int left = stack.peek();
                        int right = i;
                        int h = newHeights[mid];
                        int w = right-left-1;
                        res = Math.max(res,h*w);
                    }
                }
                stack.push(i);
            }
        }
        return res;
    }
}