67.add-binary2.java

/*
 * @lc app=leetcode id=67 lang=java
 *
 * [67] Add Binary
 *
 * https://leetcode.com/problems/add-binary/description/
 *
 * algorithms
 * Easy (37.99%)
 * Total Accepted:    278K
 * Total Submissions: 731.5K
 * Testcase Example:  '"11"\n"1"'
 *
 * Given two binary strings, return their sum (also a binary string).
 * 
 * The input strings are both non-empty and contains only characters 1 or 0.
 * 
 * Example 1:
 * 
 * 
 * Input: a = "11", b = "1"
 * Output: "100"
 * 
 * Example 2:
 * 
 * 
 * Input: a = "1010", b = "1011"
 * Output: "10101"
 * 
 */
public class Solution {
    public String addBinary(String a, String b) {
        StringBuilder sb = new StringBuilder();
        int i = a.length() - 1, j = b.length() -1, carry = 0;
        while (i >= 0 || j >= 0) {
            int sum = carry;
            if (j >= 0) sum += b.charAt(j--) - '0';
            if (i >= 0) sum += a.charAt(i--) - '0';
            sb.append(sum % 2);
            carry = sum / 2;
        }
        if (carry != 0) sb.append(carry);
        return sb.reverse().toString();
    }
}