java大数加法

java大数加法

上次屁颠屁颠的跑了1k多公里去ucweb面试，本来自我感觉还不错的，可是还是很郁闷得就给打回来了……
     其中有个笔试题是求两个大数（比如100位）的和，我当时知道怎么做，思路也没问题，但是在纸上划的时候估计有些细节没处理好，比如判断输入等。回来后自己重新做了一遍，把它贴出来共享，奉献给即将面试的兄弟伙，有什么问题请拍砖。
/** * @author zhangyong * */ public class BigInt { /** * @param args */ public static void main(String[] args) { BigInt b = new BigInt(); b.add("999", "999"); } public String add(String a, String b) { //检查输入 if (!a.matches("\\d+") || !a.matches("\\d+")) { return null; } final int BASE = 10;//10进制 int lenA = a.length();//加数的长度 int lenB = b.length();//被加数的长度 int maxLen, partialSum, carry = 0;//大数的长度，和，进位 maxLen = (lenA > lenB) ? lenA : lenB; StringBuffer sum = new StringBuffer(); int temA, temB = 0; for (int i = 0; i < maxLen; i++) { if (i >= lenA) { temA = 0; } else { temA = Integer.valueOf(a.charAt(lenA - i - 1) - 48); } if (i >= lenB) { temB = 0; } else { temB = Integer.valueOf(b.charAt(lenB - i - 1) - 48); } partialSum = temA + temB + carry; carry = partialSum / BASE; sum.append(partialSum % BASE); } if (carry == 1) sum.append(carry); System.out.println(a + "+" + b + "=" + sum.reverse().toString()); return sum.reverse().toString(); } }
    网络上还有另外一个版本：
package test; public class VeryBigNumAdd { public static void main(String[] args) { VeryBigNumAdd vbn = new VeryBigNumAdd(); String a = "123453243455535634535252345234677576252241234123523453664563634"; String b = "123453243455535634535252345234677576252241234123523453664563634"; String result = vbn.doAdd(a, b); System.out.println("result:" + result); } String doAdd(String a, String b) { String str = ""; int lenA = a.length(); int lenB = b.length(); int maxLen = (lenA > lenB) ? lenA : lenB; int minLen = (lenA < lenB) ? lenA : lenB; String strTmp = ""; for (int i = maxLen - minLen; i > 0; i--) { strTmp += "0"; } // 把长度调整到相同 if (maxLen == lenA) { b = strTmp + b; } else a = strTmp + a; int JW = 0;// 进位 for (int i = maxLen - 1; i >= 0; i--) { int tempA = Integer.parseInt(String.valueOf(a.charAt(i))); int tempB = Integer.parseInt(String.valueOf(b.charAt(i))); int temp; if (tempA + tempB + JW >= 10 && i != 0) { temp = tempA + tempB + JW - 10; JW = 1; } else { temp = tempA + tempB + JW; JW = 0; } str = String.valueOf(temp) + str; } return str; } }
   这又是一个版本：
import java.util.ArrayList; import java.util.Collections; public class VeryLongInt { ArrayList digits; // Postcondition: The VeryLongInt is empty. public VeryLongInt() { final int INITIAL_CAPACITY = 500; digits = new ArrayList(INITIAL_CAPACITY); } // default constructor // Precondition: n >= 0. // Postcondition: The VeryLongInt has been initialized from the // not-very-long int n. public VeryLongInt(int n) { final int BASE = 10; digits = new ArrayList(); do { digits.add(new Integer(n % BASE)); n = n / BASE; } // while while (n > 0); // digits is now in reverse order, so we reverse: Collections.reverse(digits); } // constructor // Postcondition: The number of elements in the VeryLongInt has // been returned. public int size() { return digits.size(); } // method size // Precondition: The string s consists of a sequence of characters // with non-digit characters ignored. // There are no leading zeroes, except for 0 // itself, which has a single '0'. // Postcondition: The VeryLongInt has been initialized from s. public VeryLongInt(String s) { final char LOWEST_DIGIT_CHAR = '0'; final char HIGHEST_DIGIT_CHAR = '9'; digits = new ArrayList(s.length()); char c; int digit; for (int i = 0; i < s.length(); i++) { c = s.charAt(i); if ((LOWEST_DIGIT_CHAR <= c) && (c <= HIGHEST_DIGIT_CHAR)) { digit = c - LOWEST_DIGIT_CHAR; digits.add(new Integer(digit)); } // if a digit } // for } // constructor with string parameter // Postcondition: The VeryLongInt has been returned as a string of digits. public String toString() { final String EMPTY_STRING = ""; String s = EMPTY_STRING; for (int i = 0; i < digits.size(); i++) s += digits.get(i); return s; } // method toString // Postcondition: If i >= digits.size(), 0 has been returned; else // the ith least significant digit in digits has // been returned. The least significant digit is // the 0th least significant digit. private int least(int i) { if (i >= digits.size()) return 0; else return ((Integer) (digits.get(digits.size() - i - 1))).intValue(); } // least // Postcondition: The VeryLongInt has been incremented by otherVeryLong. public void add(VeryLongInt otherVeryLong) { final int BASE = 10; int largerSize, partialSum, carry = 0; VeryLongInt sum = new VeryLongInt(); if (digits.size() > otherVeryLong.digits.size()) largerSize = digits.size(); else largerSize = otherVeryLong.digits.size(); for (int i = 0; i < largerSize; i++) { partialSum = least(i) + otherVeryLong.least(i) + carry; carry = partialSum / BASE; sum.digits.add(new Integer(partialSum % BASE)); } // for if (carry == 1) sum.digits.add(new Integer(carry)); Collections.reverse(sum.digits); digits = sum.digits; } // method add // Postcondition: A copy of the calling object has been returned. public Object clone() { VeryLongInt temp = new VeryLongInt(); temp.digits = (ArrayList) digits.clone(); return temp; } // method clone // Postcondition: true has been returned if the // value of the VeryLongInt is less than the value // of otherVeryLong. Otherwise, false // has been returned. public boolean less(VeryLongInt otherVeryLong) { if (digits.size() < otherVeryLong.digits.size()) return true; if (digits.size() > otherVeryLong.digits.size()) return false; for (int i = 0; i < digits.size(); i++) { if (((Integer) (digits.get(i))).intValue() < ((Integer) (otherVeryLong.digits .get(i))).intValue()) return true; if (((Integer) (digits.get(i))).intValue() > ((Integer) (otherVeryLong.digits .get(i))).intValue()) return false; } // for return false; // the two objects have the same value } // method less // Postcondition: true has been returned if the value of the VeryLongInt // is greater than the value of otherVeryLong. Otherwise, // false has been returned. public boolean greater(VeryLongInt otherVeryLong) { return otherVeryLong.less(this); } // method greater // Postcondition: true has been returned if the value of the VeryLongInt // is equal to the value of otherVeryLong. Otherwise, // false has been returned. public boolean equals(VeryLongInt otherVeryLong) { return !less(otherVeryLong) && !otherVeryLong.less(this); } // method equals // Precondition: n > 0. // Postcondition: The calling object contains the nth Fibonacci // number. public void fibonacci(int n) { VeryLongInt previous = new VeryLongInt(1), current = new VeryLongInt(1), temp = new VeryLongInt(); digits.clear(); if (n <= 2) digits.add(new Integer(1)); else { for (int i = 3; i <= n; i++) { temp = (VeryLongInt) current.clone(); current.add(previous); previous = temp; } // for digits = current.digits; } // else } // method fibonacci } // class VeryLongInt

By：残梦追月