计算大数之和

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/**
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 * @description 计算大数之和
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 * 
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 * @param {string} a
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 * @param {string} b
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 * @return {string}
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 * @example
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 * sum('111', '19')  // '130'
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 */
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function sum(a, b) {
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    // 最大位数
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    const len = Math.max(a.length, b.length)
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    // 不够位数前面补0
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    a = a.padStart(len, '0')      // 1 1 1
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    b = b.padStart(len, '0')      // 0 1 9
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    // 进位
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    let carry = 0
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    // 结果
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    let result = ''
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    // 从后面往前遍历
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    for (let i = len - 1; i >= 0 ; i--) {
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        const sum = +a[i] + +b[i] + carry
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        // 字符不断累加 
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        // i = 2, sum = 1 + 9 + 0 = 10, result = 10 % 10 + '' = '0', carry = 1
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        // i = 1, sum = 1 + 1 + 1 = 3, result = 3 % 10  + '0'= '30', carry = 0
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        // i = 0, sum = 1 + 0 + 0 = 1, result = 1 % 10 + '30' = '130', carry = 1
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        // 最终结果为 130
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        result = sum %10 + result
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        // 满 10 进 一
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        carry = Math.floor(sum / 10)
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    }
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    // 如果还有进位
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    if (carry > 0) {
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        result = carry + result
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    }
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    return result
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}
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console.log(sum('111','19'))
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console.log(sum('111111111111111111111119999', '911111111111111111111191111'))

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130
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1022222222222222222222311110