题解 | #大数加法#

import java.util.*;

public class Solution {

    public String solve (String s, String t) {
        StringBuilder stringBuilder = new StringBuilder();
        int carry = 0;
        for (int i = s.length() - 1, j = t.length() - 1; i >= 0 || j >= 0; i--, j--) {
            int x = i >= 0 ? s.charAt(i) - '0' : 0;
            int y = j >= 0 ? t.charAt(j) - '0' : 0;
            int summary = x + y + carry;
            stringBuilder.insert(0,summary % 10);
            carry = summary / 10;
        }

        if (carry != 0) {
            stringBuilder.insert(0, carry);
        }

        return stringBuilder.toString();
    }
}