java实现二叉树的遍历（递归和非递归）

源码地址：
https://github.com/TimePickerWang/aimed-at-offer/blob/master/java%E6%BA%90%E7%A0%81/TreeInfo.java

现有一颗如下图所示的二叉树：

其遍历的各种方式如下：

构造一颗如下图所示的二叉树，用java实现其前序，中序，后序遍历

注意二叉树节点的定义如下：

public class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;

    TreeNode(int x) {
        val = x;
    }
}

递归实现：

// 用递归的方法进行先序遍历
    public void qinaxuDigui(TreeNode treeNode) {
        qianxuNumList.add(treeNode.val);
        if (treeNode.left != null) {
            qinaxuDigui(treeNode.left);
        }
        if (treeNode.right != null) {
            qinaxuDigui(treeNode.right);
        }
    }

    // 用递归的方法进行中序遍历
    public void zhongxuDigui(TreeNode treeNode) {
        if (treeNode.left != null) {
            zhongxuDigui(treeNode.left);
        }
        zhongxuNumList.add(treeNode.val);
        if (treeNode.right != null) {
            zhongxuDigui(treeNode.right);
        }
    }

    // 用递归的方法进行后序遍历
    public void houxuDigui(TreeNode treeNode) {
        if (treeNode.left != null) {
            houxuDigui(treeNode.left);
        }
        if (treeNode.right != null) {
            houxuDigui(treeNode.right);
        }
        houxuNumList.add(treeNode.val);
    }

非递归实现：

// 用非递归的方法进行先序遍历
    public void qinaxuFeiDigui(TreeNode treeNode) {
        Stack<TreeNode> stack = new Stack<TreeNode>();
        while (treeNode != null || !stack.isEmpty()) {
            while (treeNode != null) {
                qianxuNumList.add(treeNode.val);
                stack.push(treeNode);
                treeNode = treeNode.left;
            }
            if(!stack.isEmpty()){
                treeNode = stack.pop();
                treeNode = treeNode.right;
            }
        }
    }

    // 用非递归的方法进行中序遍历
    public void zhongxuFeiDigui(TreeNode treeNode) {
        Stack<TreeNode> stack = new Stack<TreeNode>();
        while (treeNode != null || !stack.isEmpty()) {
            while (treeNode != null) {
                stack.push(treeNode);
                treeNode = treeNode.left;
            }
            if (!stack.isEmpty()) {
                treeNode = stack.pop();
                zhongxuNumList.add(treeNode.val);
                treeNode = treeNode.right;
            }
        }
    }

    // 用非递归的方法进行后序遍历
    public void houxuFeiDigui(TreeNode treeNode) {
        Stack<TreeNode> stack = new Stack<TreeNode>();
        while (treeNode != null || !stack.isEmpty()) {
            while (treeNode != null) {
                stack.push(treeNode);
                treeNode = treeNode.left;
            }
            boolean tag = true;
            TreeNode preNode = null;  // 前驱节点
            while (!stack.isEmpty() && tag == true) {
                treeNode = stack.peek();
                if (treeNode.right == preNode) { // 之前访问的为空节点或是栈顶节点的右子节点
                    treeNode = stack.pop();
                    houxuNumList.add(treeNode.val);
                    if (stack.isEmpty()) {
                        return;
                    } else {
                        preNode = treeNode;
                    }
                } else {
                    treeNode = treeNode.right;
                    tag = false;
                }
            }
        }
    }

运行结果如下：