536. Construct Binary Tree from String

You need to construct a binary tree from a string consisting of parenthesis and integers.

The whole input represents a binary tree. It contains an integer followed by zero, one or two pairs of parenthesis. The integer represents the root's value and a pair of parenthesis contains a child binary tree with the same structure.

You always start to construct the left child node of the parent first if it exists.

 

Example 1:

Input: s = "4(2(3)(1))(6(5))"
Output: [4,2,6,3,1,5]

Example 2:

Input: s = "4(2(3)(1))(6(5)(7))"
Output: [4,2,6,3,1,5,7]

Example 3:

Input: s = "-4(2(3)(1))(6(5)(7))"
Output: [-4,2,6,3,1,5,7]

class Solution {
    public TreeNode str2tree(String s) {
        return helper(s, 0, s.length() - 1);
    }
    private TreeNode helper(String s, int start, int end) {
        if (start > end) return null;
        int leftBraceIndex = s.indexOf('(', start);
        if (leftBraceIndex == -1 || leftBraceIndex > end) {
            String strValue = s.substring(start, end + 1);
            return new TreeNode(Integer.valueOf(strValue));
        }
        int rootValue = Integer.valueOf(s.substring(start, leftBraceIndex));
        TreeNode root = new TreeNode(rootValue);
        int k = leftBraceIndex; 
        int count = 0;
        for (;k <= end; k++) {
            if (s.charAt(k) == '(') {
                count++;
            } else if (s.charAt(k) == ')') {
                count--;
            }
            
            if (count == 0) {
                break;
            }
        }
        root.left = helper(s, leftBraceIndex + 1, k - 1);
        root.right = helper(s, k + 2, end - 1);
        return root;
    }
}