【实战】ACM 选手图解 LeetCode 有效的括号

【实战】ACM 选手图解 LeetCode 删除字符串中的所有相邻重复项 图解 LeetCode 1047:删除字符串中的所有相邻重复项 阅读详情

大家好呀,我是帅蛋。

今天来判断括号是否有效,常见题。

不知道说啥,开整吧。

800b54ae3d38699d34a6850c59e32f2

LeetCode 20:有效的括号

题意

给定一个只包括 ‘(’,’)’,’{’,’}’,’[’,’]’ 的字符串 s ,判断字符串是否有效。

示例

输入:s = “()[]{}”

输出:true

输入:s = “([)]”

输出:false

提示

有效字符串需满足:

  • 左括号必须用相同类型右括号闭合
  • 左括号必须以正确顺序闭合

1 <= s.length <= 10^4

s 仅由括号 () [] {} 组成

题目解析

括号匹配是使用栈解决的经典问题,难度简单。

还不太会的臭宝看下面链接,本蛋写的:

呔!“栈”住,队列!

看会了栈,这道题基本就会了大半,剩下小半就是动动小脑袋瓜,稍微思考。

这题就是维护一个栈,碰到左括号就入栈,碰到右括号,取出栈顶元素,判断栈顶元素和右括号是否匹配。

如果不匹配,返回 false,如果匹配则重复上述步骤,直到遍历完字符串。

需要注意的是,有两种特殊情况也是返回 false:

  • 遍历完整个字符串,栈不为空,说明多了左括号。
  • 碰到右括号,但此时栈为空,说明多了右括号。

抛除上面的情况,其余的都返回 true。

图解

假设 s = “()[]{}”

首先初始化。

# 初始化
stack = []

pairs = {
    ')': '
【实战】ACM 选手图解 LeetCode 逆波兰表达式求值 图解 LeetCode 150:逆波兰表达式求值 阅读详情

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nn.output = 'softmax'; % use softmax outputopts.numepochs = 5; % Number of full sweeps through dataopts.batchsize = 1000; % Take a mean gradient step over this many samplesopts.plot = 1; % enable plottingnn = nntrain(nn, tx, ty, opts, vx, vy); % nntrain takes validation set as last two arguments (optionally)[er, bad] = nntest(nn, test_x, test_y);assert(er < 0.1, 'Too big error');```[![Bitdeli Badge](https://d2weczhvl823v0.cloudfront.net/rasmusbergpalm/deeplearntoolbox/trend.png)](https://bitdeli.com/free "Bitdeli Badge")

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