LeetCode Add Binary

原题链接在这里: https://leetcode.com/problems/add-binary/

这道题与Plus One相似。都是维护一个currentDigit(%) 和一个carryDigit(/). 

Time O(max(m,n)), Space O(max(m,n)), m = a.length(), n = b.length()

Note: 1. 注意是从String的尾部算起，所以注意i 和index的关系；

2. 若是算到最后，carryDigit还是1的话就是有一个最后的进位，要在res的首位加起来。

AC Java:

public class Solution {
    public String addBinary(String a, String b) {
        if(a==null && b==null && a.length()==0 && b.length()==0){
            return "";
        }
        
        int len1 = a.length();
        int len2 = b.length();

        StringBuilder res = new StringBuilder();
        int carryDigit = 0;
        int currentDigit;
        int i = 0;
        while(i<len1 && i<len2){
            currentDigit = (Character.getNumericValue(a.charAt(len1-1-i)) + Character.getNumericValue(b.charAt(len2-1-i)) + carryDigit)%2;
            carryDigit = (Character.getNumericValue(a.charAt(len1-1-i)) + Character.getNumericValue(b.charAt(len2-1-i)) + carryDigit)/2;
            if(currentDigit == 0){
                res.insert(0,"0");
            }else{
                res.insert(0,"1");
            }
            i++;
        }
        
        if(len1 > len2){
            while(i<len1){
                currentDigit = (Character.getNumericValue(a.charAt(len1-1-i)) + carryDigit)%2;
                carryDigit = (Character.getNumericValue(a.charAt(len1-1-i)) + carryDigit)/2;
                if(currentDigit == 0){
                    res.insert(0,"0");
                }else{
                    res.insert(0,"1");
                }
                i++;
            }
        }
        if(len1 < len2){
            while(i<len2){
                currentDigit = (Character.getNumericValue(b.charAt(len2-1-i)) + carryDigit)%2;
                carryDigit = (Character.getNumericValue(b.charAt(len2-1-i)) + carryDigit)/2;
                if(currentDigit == 0){
                    res.insert(0,"0");
                }else{
                    res.insert(0,"1");
                }
                i++;
            }
        }
        if(carryDigit != 0){
                res.insert(0,"1");
        }
        return res.toString();
    }
}