本方法的思路為：
一：檢查了輸入的合法性（非空，無非法字符）
二：檢查輸入是否可以進行簡單計算（一個數為 0，1，+1，-1）
三：去掉輸入最前面可能有的正負符號，并判斷輸出的正負
四：將輸入的值分成4位一截（分的長度太短，性能太差，長度太長，精度容易降低）
五：遍歷相乘得到最終數組（這里用了遞歸）
六：遍歷最終數組，拼接最終的數（不建議用join,因為數組中的元素可能小于四位，拼接時會丟失0）
七：將正負符號與最終的數拼接輸出
代碼如下：
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<!DOCTYPE html> <html> <head><title></title><script type="text/javascript">var num1 = "";var num2 = "";var finallyArr;/** 有兩個用字符表示的非常大的大整數，算出他們的乘積，也是用字符串表示，不能用系統自帶的大整數類型。* 輸入例子72106547548473106236 982161082972751393* 輸出例子7082024482963453804084865646610598674870820244829634538040848656466105986748*/function getResult(){finallyArr = new Array();num1 = document.getElementById("num1").value;num2 = document.getElementById("num2").value; if(!checkNum(num1)){alert("輸入:num1 不合法！");return;}if(!checkNum(num2)){alert("輸入:num2 不合法！");return;}//特殊情況的簡單算法if(doSimpleWay()){return;}//獲取最終值的正負符號var symbol = initSymbol();//將輸入的值分成4位一小截的數組var numArr1 = getNumArr(num1,new Array());var numArr2 = getNumArr(num2,new Array());//遍歷相乘相加得到最終的數組for(var i = 0; i < numArr1.length; i++){for(var j = 0; j < numArr2.length; j++){updateFinallyArr(i + j , numArr1[i] * numArr2[j]); }}//倒序遍歷數組獲取最終結果var resultStr = finallyArr.pop();for(var i = finallyArr.length; i >= 0; i--){if(finallyArr[i] >= 1000){resultStr += "" + finallyArr[i];}else if(finallyArr[i] < 1000){resultStr += "0" + finallyArr[i];}else if(finallyArr[i] < 100){resultStr += "00" + finallyArr[i];}else if(finallyArr[i] < 10){resultStr += "000" + finallyArr[i];}else if(finallyArr[i] == 0){resultStr += "0000";}}document.getElementById("RESULT").innerText = symbol + resultStr;}/*** 根據數值以及數值的位置更新最終的數組*/function updateFinallyArr(i , num){var old = finallyArr[i];if(old){num = num + old;}if(num > 9999){finallyArr[i] = num % 10000;updateFinallyArr(i + 1 , Math.floor(num/10000) ); }else{finallyArr[i] = num;}}/*** 處理輸入字符串的正負號，并判斷最終值的正負號*/function initSymbol(){if(num1.substring(0,1) == "-"){num1 = num1.substring(1,num1.length); if(num2.substring(0,1) == "-"){num2 = num2.substring(1,num2.length);}else if(num2.substring(0,1) == "+"){num2 = num2.substring(1,num2.length);return "-";}else{return "-";}}else if(num1.substring(0,1) == "+"){num1 = num1.substring(1,num1.length); if(num2.substring(0,1) == "-"){num2 = num2.substring(1,num2.length);return "-";}else if(num2.substring(0,1) == "+"){num2 = num2.substring(1,num2.length); }}return "";}/*** 特殊情況的簡單算法*/function doSimpleWay(){if(num1 == "0" || num2 == "0"){document.getElementById("RESULT").innerText = "0";return true;}else if(num1 == "1" || num1 == "+1"){document.getElementById("RESULT").innerText = num2;return true;}else if(num2 == "1" || num2 == "+1"){document.getElementById("RESULT").innerText = num1;return true;}else if(num1 == "-1"){if(num2.substring(0,1) == "-" ){document.getElementById("RESULT").innerText = num2.substring(1,num2.length);}else if(num2.substring(0,1) == "+" ){document.getElementById("RESULT").innerText = "-" + num2.substring(1,num2.length);}else{document.getElementById("RESULT").innerText = "-" + num2;}return true;}else if(num2 == "-1"){if(num1.substring(0,1) == "-" ){document.getElementById("RESULT").innerText = num1.substring(1,num1.length);}else if(num1.substring(0,1) == "+" ){document.getElementById("RESULT").innerText = "-" + num1.substring(1,num1.length);}else{document.getElementById("RESULT").innerText = "-" + num1;}return true;}return false;}/*** 獲取4位一小截的數組*/function getNumArr(num,arr){var len = num.length;if(len > 4){arr.push(num.substring(len-4,len) * 1);num = num.substring(0,len-4);return getNumArr(num,arr);}else{arr.push(num * 1);return arr;}}/*** 檢查輸入數字的正確性，非空、無不合法字符*/function checkNum(num){if(!num){return false;}var str = num.replace(/[0-9]/g,"");if(str == "" || (str == "-" && num.substring(0,1) =="-" && num.length > 1) || (str == "+" && num.substring(0,1) =="+" && num.length > 1) ){if(num.length > 1000){return false;}return true;}else{return false;}}</script> </head> <body>num1:<input type="text" id="num1" name=""
value="7210654754847310623672106547548473106236721065475484731062367210654754847310623672106547548473106236" style="width:80%"></br>num2:<input type="text" id="num2" name=""
value="9821610829727513939821610829727513939821610829727513939821610829727513939821610829727513939821610829" style="width:80%"></br><input type="button" id="num2" value="計算" onclick="getResult()"></br><textarea id = "RESULT" rows="5" style="width:80%"></textarea> </body> </html>
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