394_decode_string

Leetcode 394: Decode String
Detailed video explanation: https://youtu.be/yaCRdWMq4A4
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C++:
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class Solution {
    string decodeString(string& s, int& i) {
        string result;
        while(i < s.length() && s[i] != ']'){
            if(isdigit(s[i])){
                int k = 0;
                while(i < s.length() && isdigit(s[i]))
                    k = k*10 + s[i++] - '0';
                i++;
                string r = decodeString(s, i);
                while(k-- > 0)
                    result += r;
                i++;
            } else
                result += s[i++];
        }
        return result;
    }
public:
    string decodeString(string s) {
        int i = 0;
        return decodeString(s, i);
    }
};



Java:
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class Solution {
    int i = 0;
    public String decodeString(String s) {
        StringBuilder result = new StringBuilder();
        while(i < s.length() && s.charAt(i) != ']'){
            if(Character.isDigit(s.charAt(i))){
                int k = 0;
                while(i < s.length() && Character.isDigit(s.charAt(i)))
                    k = k*10 + s.charAt(i++) - '0';
                i++;
                String r = decodeString(s);
                while(k-- > 0)
                    result.append(r);
                i++;
            } else
                result.append(s.charAt(i++));
        }
        return result.toString();
    }
}