Deserialize Binary Tree (Mutation)

Question

Given a string, deserialize the string into tree. Each root node will be followed by () with its child nodes inside. String "4(5,6)" represents the tree:

 4
/  \
5  6

String "4(5(7,8),9)" represents the tree:

        4
      /   \
     5     9
   /   \
   7   8

Solution

int index = 0; // global variable to record the processing character during recursion

public TreeNode deserialize(String s) {
    return helper(s)[0];
}

private TreeNode[] helper(String s) { 
    // Always skip the last ')' before return;
    if(index==s.length()) return null;
    TreeNode n1 = null;
    TreeNode n2 = null;
    int i=st;

    //////////////////////////n1
    int val = 0;
    if(i<s.length() && Character.isDigit(s.charAt(i))) {
        while(i<s.length() && Character.isDigit(s.charAt(i))) {
            val = val*10+s.charAt(i)-'0';
            i++;
        }
        n1 = new TreeNode(val);
    }

    if(i<s.length() && s.charAt(i)=='(') { // find subchild of n1
        index = i+1;
        TreeNode[] child = helper(s);
        n1.left = child[0];
        n1.right = child[1];
    }
    i = index; // reset i from last recursion

    if(i<s.length() && s.charAr(i)==',') i++; //skip the ','

    //////////////////////////n2
    val = 0;
    if(i<s.length() && Character.isDigit(s.charAt(i))) {
        while(i<s.length() && Character.isDigit(s.charAt(i))) {
            val = val*10+s.charAt(i)-'0';
            i++;
        }
        n2 = new TreeNode(val);
    }


    if(i<s.length() && s.charAt(i)=='(') { // find subchild of n1
        index = i+1;
        TreeNode[] child = helper(s);
        n2.left = child[0];
        n2.right = child[1];
    }
    i = index; // reset i from last recursion

    //////////////////////////Return two child nodes
    if(i<s.length() && s.charAr(i)==')') index = i+1;
    return new TreeNode[]{n1,n2};
}