二叉树的遍历-先序遍历、中序遍历、后序遍历

二叉树的先序遍历，就是先输出根结点，再遍历左子树，最后遍历右子树，遍历左子树和右子树的时候，同样遵循先序遍历的规则。

    public void preOrder(TreeNode treeNode){
        if(treeNode == null){
            return;
        }
        System.out.print(" " + treeNode.value);
        preOrder(treeNode.left);
        preOrder(treeNode.right);
    }

二叉树的中序遍历，就是先递归中序遍历左子树，再输出根结点的值，再递归中序遍历右子树。

    
    public void inOrder(TreeNode treeNode){
        if(treeNode == null){
            return;
        }
        inOrder(treeNode.left);
        System.out.print(" " + treeNode.value);
        inOrder(treeNode.right);
    }

二叉树的后序遍历，就是先递归后序遍历左子树，再递归后序遍历右子树，最后输出根结点的值。

 
    public void postOrder(TreeNode treeNode){
        if(treeNode == null){
            return;
        }
        postOrder(treeNode.left);
        postOrder(treeNode.right);
        System.out.print(" " + treeNode.value);
    }

案例：

public class TreeDemo {

    public class TreeNode{
        private Integer value;
        private TreeNode right;
        private TreeNode left;

        public TreeNode() {
        }

        public TreeNode(Integer value, TreeNode left, TreeNode right) {
            this.value = value;
            this.right = right;
            this.left = left;
        }

        public Integer getvalue() {
            return value;
        }

        public void setvalue(Integer value) {
            this.value = value;
        }

        public TreeNode getRight() {
            return right;
        }

        public void setRight(TreeNode right) {
            this.right = right;
        }

        public TreeNode getLeft() {
            return left;
        }

        public void setLeft(TreeNode left) {
            this.left = left;
        }
    }

    public static void main(String[] args) {
        TreeDemo treeDemo = new TreeDemo();
        TreeNode tree = treeDemo.getTree();
        System.out.println("先序遍历:");
        treeDemo.preOrder(tree);
        System.out.println();
        System.out.println("中序遍历:");
        treeDemo.inOrder(tree);
        System.out.println();
        System.out.println("后序遍历:");
        treeDemo.postOrder(tree);
    }

    public TreeNode getTree(){
        TreeNode treeNode = new TreeNode();
        treeNode.setvalue(13);
        treeNode.setLeft(new TreeNode(10,
                new TreeNode(7,
                new TreeNode(3,null,null),new TreeNode(4,null,null)),
                new TreeNode(12,new TreeNode(11,null,null),null)));


        treeNode.setRight(new TreeNode(15,
                new TreeNode(14,null,null),
                new TreeNode(17,null,null)));
        return treeNode;
    }

    
    public void preOrder(TreeNode treeNode){
        if(treeNode == null){
            return;
        }
        System.out.print(" " + treeNode.value);
        preOrder(treeNode.left);
        preOrder(treeNode.right);
    }

    
    public void inOrder(TreeNode treeNode){
        if(treeNode == null){
            return;
        }
        inOrder(treeNode.left);
        System.out.print(" " + treeNode.value);
        inOrder(treeNode.right);
    }

    
    public void postOrder(TreeNode treeNode){
        if(treeNode == null){
            return;
        }
        postOrder(treeNode.left);
        postOrder(treeNode.right);
        System.out.print(" " + treeNode.value);
    }
}

结果如下：