@@ -47,6 +47,96 @@ class Solution(object):
4747 return True
4848```
4949
50+ ## 84. 柱状图中最大的矩形
51+ 52+ [ 原题链接] ( https://leetcode-cn.com/problems/largest-rectangle-in-histogram/ )
53+ 54+ ### 解法一
55+ 56+ - 单调栈
57+ - 逐个遍历
58+ - 结果:超时
59+ 60+ ``` python
61+ class Solution :
62+ def largestRectangleArea (self heights : List[int ]) -> int :
63+ stack = [] # 单调递增栈
64+ ans = 0
65+ for h in heights:
66+ stack.append(h)
67+ # if len(stack) == 0:
68+ # stack.append(h)
69+ # else:
70+ # while stack[-1] > h:
71+ # # 对比栈顶元素
72+ # top = stack[-1]
73+ # # 左侧更大的数字被化为小数
74+ # if top > h:
75+ # stack[-1] = h
76+ # stack.append(h)
77+ # print(stack)
78+ # 计算实际面积
79+ stack_length = len (stack)
80+ for i in range (stack_length):
81+ # element = stack[i]
82+ if stack[i] > h:
83+ stack[i] = h
84+ area = (stack_length - i) * stack[i]
85+ ans = max (ans, area)
86+ # print(ans)
87+ 88+ return ans
89+ ```
90+ 91+ ### 解法二
92+ 93+ 面积计算方式:
94+ 95+ - 找到 ` heights[i] ` 左侧第一个小于它的元素位置 ` left `
96+ - 找到 ` heights[i] ` 右侧第一个小于它的元素位置 ` right `
97+ - ` area = (right - left - 1) * heights[i] `
98+ 99+ 利用维护单调递增栈得出元素的左右边界:
100+ 101+ - 左侧边界(左侧第一个小于当前位置的元素)可以在元素入栈时确认
102+ - 右侧边界(右侧第一个小于当前位置的元素)可以在元素出栈时确认
103+ 104+ ``` python
105+ class Solution :
106+ def largestRectangleArea (self heights : List[int ]) -> int :
107+ # 找左侧第一个小于 h 的位置 left
108+ # 右侧第一个小于 h 的位置 right
109+ # area = (right - left) * h
110+ 111+ # 维护单调栈
112+ length = len (heights)
113+ stack = []
114+ lefts = [- 1 for _ in range (length)]
115+ rights = [length for _ in range (length)]
116+ for i in range (length):
117+ h = heights[i]
118+ if len (stack) == 0 :
119+ stack.append((i, h))
120+ else :
121+ # 维护单调栈
122+ while len (stack) > 0 and stack[- 1 ][1 ] >= h:
123+ # 弹出时能确认右侧最小元素
124+ min_element = stack.pop()
125+ rights[min_element[0 ]] = i
126+ # 入栈,确认左侧最小元素
127+ if len (stack) > 0 :
128+ lefts[i] = stack[- 1 ][0 ]
129+ stack.append((i, h))
130+ 131+ ans = 0
132+ for i in range (length):
133+ area = (rights[i] - lefts[i] - 1 ) * heights[i]
134+ # print((rights[i] - lefts[i] + 1))
135+ ans = max (area, ans)
136+ 137+ return ans
138+ ```
139+ 50140## 150. 逆波兰表达式求值
51141
52142[ 原题链接] ( https://leetcode-cn.com/problems/evaluate-reverse-polish-notation/ )
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