判断一个树是否为平衡二叉树&&二分搜索树 && 完全二叉树

   public boolean isBalanced(TreeNode root) {
       if (root == null) return true;
       if (Math.abs(treeHeight(root.right)-treeHeight(root.left ))>1) return false;
       return isBalanced(root.left)&&isBalanced(root.right);
    }

    public int treeHeight(TreeNode root){
        if (root == null) return 0;
        return 1+Math.max(treeHeight(root.left),treeHeight(root.right));
    }

public boolean isBalanced(TreeNode root) {
    return process(root).isBalance;
}
    
public static class ReturnData{
    boolean isBalance;
    int height;
    public ReturnData(boolean isBalance, int height) {
        this.isBalance = isBalance;
        this.height = height;
    }
}

public static ReturnData process(TreeNode root){
     if (root == null ) return new ReturnData(true,0);
     ReturnData left = process(root.left);
     if (!left.isBalance) return new ReturnData(false,0);
     ReturnData right = process(root.right);
     if (!right.isBalance) return new ReturnData(false,0);
     if ( Math.abs(left.height - right.height)>1) return new ReturnData(false,0);
     return new ReturnData(true,Math.max(left.height,right.height)+1);
}

   public boolean isValidBST(TreeNode root) {
        if (root == null) return true;
        Stack<TreeNode> stack = new Stack<>();
        TreeNode pre = null;
        while ( root != null || !stack.isEmpty()){
            if (root == null){
                TreeNode temp = stack.pop();
                if (pre != null){
                    if (temp.val <= pre.val) return false;
                }
                pre = temp;
                root = temp.right;
            }else {
                stack.push(root);
                root = root.left;
            }
        }
        return true;
    }

    public boolean isCompleteTree(TreeNode root) {
        if (root == null ) return true;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        boolean leaf = false;
        while (!queue.isEmpty()) {
            TreeNode temp = queue.poll();
            if ((temp.left == null && temp.right != null) || 
                (leaf && (temp.left != null || temp.right != null))){
                return false;
            }
            if (temp.left == null || temp.right == null) {
                leaf = true;
            }
            if ( temp.left != null){
                queue.offer(temp.left);
            }
            if ( temp.right != null ){
                queue.offer(temp.right);
            }
        }
        return true;
    }