目录

解析数学表达式

字符串解码/智能重复

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解析数学表达式

const stock = []; // 先进后出，每一次出栈，即一对 ()
const parenthesesPairPosition = {}

// 剔除两侧空格
const removeBlank = (expression, l, r) => {
    while (expression[l] === ' ') {
        l++
    }
    
    while (expression[r] === ' ') {
        r--
    }
    
    return [l, r]
}

// 剔除两侧小括号
const removeParentheses = (l, r) => {
    if (parenthesesPairPosition[l] === r) return [++l, --r]
    
    return [l, r]
}

const parse = (expression, l = 0, r = expression.length - 1, skipSearchTimeOrDivide = false) => {
    let isNumber = true;
    let parenthesesDep = 0; // 记录小括号深度
    let firstTimeOrDivideOperator = null; // 记录遇到的第一个 * / 运算符
    let firstTimeOrDivideOperatorIdx = null; // 记录遇到的第一个 * / 运算符的位置
    
    [l, r] = removeBlank(expression, l, r);
    [l, r] = removeParentheses(l, r);

    for (let i = r; i >= l; i--) {
        const v = expression[i];
        
        if (v === ')') {
            stock.push(i)
            parenthesesDep++
        } else if (v === '(') {
            const last = stock.pop()
            parenthesesPairPosition[i] = last
            parenthesesDep--
        }
        
        // skipSearchTimeOrDivide 为 true 表示表达式是连续的 * /
        if (skipSearchTimeOrDivide && firstTimeOrDivideOperator) {
            return {
                type: firstTimeOrDivideOperator,
                left: parse(expression, l, firstTimeOrDivideOperatorIdx - 1, true),
                right: parse(expression, firstTimeOrDivideOperatorIdx + 1, r),
            }
        }

        if (i === l) {
          if (isNumber) {
            return {
              type: 'number',
              value: Number(expression.slice(l, r + 1).trim()),
            };
          }
          
          if (parenthesesPairPosition[i] === r) {
              return parse(expression, l + 1, r - 1)
          }

          // * / 拆分，需要遍历到最左侧，所里拆分逻辑写这里
          return {
              type: firstTimeOrDivideOperator,
              left: parse(expression, l, firstTimeOrDivideOperatorIdx - 1, true),
              right: parse(expression, firstTimeOrDivideOperatorIdx + 1, r),
          }
        }

        if (/[0-9.]/.test(v) || v === ' ') {
            continue;
        }
        
        isNumber = false;
        
        // parenthesesDep === 0 进行表达式分析的时候要确保是同一层级
        if (parenthesesDep === 0 && (v === '+' || v === '-')) {
            return {
                type: v,
                left: parse(expression, l, i - 1),
                right: parse(expression, i + 1, r),
            }
        }

        if (parenthesesDep === 0 && (v === '*' || v === '/') && !firstTimeOrDivideOperator) {
            firstTimeOrDivideOperator = v
            firstTimeOrDivideOperatorIdx = i
        }
    } 
}

const exec = ast => {
    const recursion = ast => {
        if (ast.type === '+') {
            return recursion(ast.left) + recursion(ast.right)
        } else if (ast.type === '-') {
            return recursion(ast.left) - recursion(ast.right)
        } else if (ast.type === '*') {
            return recursion(ast.left) * recursion(ast.right)
        } else if (ast.type === '/') {
            return recursion(ast.left) / recursion(ast.right)
        } else if (ast.type === 'number') {
            return ast.value
        }
    }
    
    return recursion(ast)
}

let expression = '(1 + (2 - (3 * 4 / 5 + 6 - (7 + 8))) + (9 - 10) * 11 + 12) + 13'

console.log(exec(parse(expression)) === eval(expression))

expression = '3.5 * 4 / 5 + 6'

console.log(exec(parse(expression)) === eval(expression))

字符串解码/智能重复

const decodeString = (s) => {
    let numStack = [];        // 存倍数的栈
    let strStack = [];        // 存 待拼接的str 的栈
    let num = 0;              // 倍数的“搬运工”
    let result = '';          // 字符串的“搬运工”
    for (const char of s) {   // 逐字符扫描
        if (!isNaN(char)) {   // 遇到数字
            num = num * 10 + Number(char); // 算出倍数
        } else if (char == '[') {  // 遇到 [
            strStack.push(result); // result串入栈
            result = '';           // 入栈后清零
            numStack.push(num);    // 倍数num进入栈等待
            num = 0;               // 入栈后清零
        } else if (char == ']') {  // 遇到 ]，两个栈的栈顶出栈
            let repeatTimes = numStack.pop(); // 获取拷贝次数
            result = strStack.pop() + result.repeat(repeatTimes); // 构建子串
        } else {                   
            result += char;        // 遇到字母，追加给result串
        }
    }
    return result;
};