java中二叉树遍历（前序、中序、后序）

一、前序遍历
1）递归实现
public void preOrderTraverse1(TreeNode root) {
if (root != null) {
System.out.print(root.val + “->”);
preOrderTraverse1(root.left);
preOrderTraverse1(root.right);
}
}

2）非递归实现
public void preOrderTraverse2(TreeNode root) {
Stack stack = new Stack<>();
TreeNode node = root;
while (node != null || !stack.empty()) {
if (node != null) {
System.out.print(node.val + “->”);
stack.push(node);
node = node.left;
} else {
TreeNode tem = stack.pop();
node = tem.right;
}
}
}

二、中序遍历
1）递归实现
public void inOrderTraverse(TreeNode root) {
if (root != null) {
inOrderTraverse(root.left);
System.out.print(root.val + “->”);
inOrderTraverse(root.right);
}
}
2）非递归实现
public void inOrderTraverse(TreeNode root) {
Stack stack = new Stack<>();
TreeNode node = root;
while (node != null || !stack.isEmpty()) {
if (node != null) {
stack.push(node);
node = node.left;
} else {
TreeNode tem = stack.pop();
System.out.print(tem.val + “->”);
node = tem.right;
}
}
}

三、后序遍历
　1）递归实现
public void postOrderTraverse(TreeNode root) {
    if (root != null) {
        postOrderTraverse(root.left);
        postOrderTraverse(root.right);
        System.out.print(root.val + "->");
    }
}
2）非递归实现
public void postOrderTraverse(TreeNode root) {
    TreeNode cur, pre = null;

    Stack<TreeNode> stack = new Stack<>();
    stack.push(root);

    while (!stack.empty()) {
        cur = stack.peek();
        if ((cur.left == null && cur.right == null) || (pre != null && (pre == cur.left || pre == cur.right))) {
            System.out.print(cur.val + "->");
            stack.pop();
            pre = cur;
        } else {
            if (cur.right != null)
                stack.push(cur.right);
            if (cur.left != null)
                stack.push(cur.left);
        }
    }
}

四、层次遍历
public void levelOrderTraverse(TreeNode root) {
if (root == null) {
return;
}
Queue queue = new LinkedList();
queue.add(root);

    while (!queue.isEmpty()) {
        TreeNode node = queue.poll();
        System.out.print(node.val + "->");

        if (node.left != null) {
            queue.add(node.left);
        }
        if (node.right != null) {
            queue.add(node.right);
        }
    }
}