创建二叉树，并先序遍历

先序创建二叉树，并从1递增赋值，先序遍历：

class TreeNode{TreeNode left;TreeNode right;int val;public TreeNode(int val){this.val = val;}public TreeNode(){}}public class BinaryTreeTest {public static void main(String[] args){TreeNode tree = new TreeNode();createTree(3, 1, tree);preOrderTraverse(tree);}public static int createTree(int N, int val, TreeNode root){int leftVal = 0;int rightVal = 0;if (N > 1){root.val = val;val++;root.left = new TreeNode();root.right = new TreeNode();leftVal = createTree(N-1, val, root.left);//leftVal++;rightVal = createTree(N-1, leftVal, root.right);return rightVal;}else{root.val = val;return ++val;}}public static void preOrderTraverse(TreeNode root){if (root == null)return;else{System.out.println(root.val);preOrderTraverse(root.left);preOrderTraverse(root.right);}}}