题解 | #大数加法#

import java.util.*;
  public class Solution{
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     * 计算两个数之和
     * @param s string字符串 表示第一个整数
     * @param t string字符串 表示第二个整数
     * @return string字符串
     */
    public static String solve (String s, String t) {
      if(s.isEmpty() || t.isEmpty()) return s.isEmpty() ? t : s;
      // 字符串反转
      StringBuilder sb1 = new StringBuilder(new StringBuilder(s).reverse());
      StringBuilder sb2 = new StringBuilder(new StringBuilder(t).reverse());
      int len1 = sb1.length();
      int len2 = sb2.length();
      StringBuilder temp = new StringBuilder();
      int cnt = 0, a, b; // 表示进位
      int i = 0, j = 0;
      for(; i < len1 && j < len2; ++i, ++j){
        a = sb1.charAt(i) - '0';
        b = sb2.charAt(j) - '0';
        temp.append((char)((a + b + cnt) % 10 + '0'));
        cnt = (a + b + cnt) / 10;
      }
      if(cnt != 0){
        if(len1 == len2) temp.append((char)(cnt + '0'));
        else if(len1 < len2){
          handler(sb2, len2, temp, cnt, j);
        }
        else {
          handler(sb1, len1, temp, cnt, i);
        }
      }
      else {
        if (len1 < len2) {
          temp.append(sb2.substring(j));
        } else if (len1 > len2) {
          temp.append(sb1.substring(i));
        }
      }
      return temp.reverse().toString();
    }

    private static void handler(StringBuilder sb1, int len1, StringBuilder temp, int cnt, int i) {
      int a;
      while(i < len1){
        a = sb1.charAt(i) - '0';
        temp.append((char)((a + cnt) % 10 + '0'));
        cnt = (a + cnt) / 10;
        i++;
      }
      if(cnt != 0){
        temp.append((char)(cnt + '0'));
      }
    }
  }