java 通过字符串建立二叉树

package mian.erchashu;

/**
 * 给定字符串建立二叉树 空节点用null表示
 * 
 * @author xuminggang
 *
 */
public class StringToBinaryTree {
          
   

	public static void main(String[] args) {
          
   
		TreeNode root = stringToTree("[3,9,20,2,null,15,7]");
		new StringToBinaryTree().preOrder(root);
	}

	public static TreeNode stringToTree(String str) {
          
   
		String[] elems = stringToArray(str);
		if (null == elems) {
          
   
			return null;
		}

		TreeNode root = createTree(elems, 0);
		return root;
	}

	/**
	 * 递归构建二叉树
	 * 
	 * @param arr
	 * @param index
	 * @return
	 */
	private static TreeNode createTree(String[] arr, int index) {
          
   
		TreeNode tn = null;
		if (index < arr.length) {
          
   
			if (arr[index].equals("null")) {
          
   
				return null;
			}
			Integer val = Integer.valueOf(arr[index]);
			tn = new TreeNode(val);
			// 构建二叉树左子树
			tn.left = createTree(arr, 2 * index + 1);
			// 构建二叉树右子树
			tn.right = createTree(arr, 2 * index + 2);
		}
		return tn;
	}

	private static String[] stringToArray(String str) {
          
   
		if (null != str && str.equals("")) {
          
   
			return null;
		}
		String[] fields = str.split(",");
		fields[0] = fields[0].substring(1);
		int lastIndex = fields.length - 1;
		fields[lastIndex] = fields[lastIndex].substring(0, 1);
		return fields;
	}

	public void preOrder(TreeNode root) {
          
   
		if (root == null) {
          
   
			return;
		}
		System.out.println(root.val);
		preOrder(root.left);
		preOrder(root.right);
	}
}

class TreeNode {
          
   

	int val;
	TreeNode left;
	TreeNode right;
	public TreeNode(int val) {
          
   
		this.val = val;
	}
}