根据二叉树创建字符串

解题思路 : 

 

代码实现 :  

public String tree2str(TreeNode root) {
        if (root == null) {
            return null;
        }
        StringBuilder sb = new StringBuilder();
        treeToString(root,sb);
        return sb.toString();
    }
    public void treeToString(TreeNode t, StringBuilder sb) {
        sb.append(t.val);
        if (t.left == null && t.right == null) {
            return;
        }
        if (t.left != null) {
            sb.append("(");
            treeToString(t.left,sb);
            sb.append(")");
        } else {
            sb.append("()");
        }
        if (t.right != null) {
            sb.append("(");
            treeToString(t.right,sb);
            sb.append(")");
        } else {
            return;
        }
    }

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