110. Balanced Binary Tree

class Solution {
    public boolean isBalanced(TreeNode root) {
        return head(root) != -1;
    }

    public int head(TreeNode root){
        if(root == null) return 0;

        int left = head(root.left);
        int right = head(root.right);
        int res = Math.abs(left - right);
        
         if(res > 1 || left == -1 || right == -1) return -1;
        return 1 + Math.max(left, right);
    }
}