二叉树的前序遍历、中序遍历、后序遍历、层序遍历

1.节点定义

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


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

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

2.中序遍历

左-根-右

public static void printTreeInOrder(TreeNode root) {
    if (root == null) {
        return;
    }
    if (root.left != null) {
        printTreeInOrder(root.left);
    }
    System.out.print(root.val + "\t");
    if (root.right != null) {
        printTreeInOrder(root.right);
    }
}

3.先序遍历

根-左-右

public static void printTreePreOrder(TreeNode root) {
    if (root == null) {
        return;
    }
    System.out.print(root.val + "\t");
    if (root.left != null) {
        printTreePreOrder(root.left);
    }
    if (root.right != null) {
        printTreePreOrder(root.right);
    }
}

4.后序遍历

左-右-根

public static void printTreePostOrder(TreeNode root) {
    if (root == null) {
        return;
    }
    if (root.left != null) {
        printTreePostOrder(root.left);
    }
    if (root.right != null) {
        printTreePostOrder(root.right);
    }
    System.out.print(root.val + "\t");

}

5.层序遍历

利用队列

public static void printTreeLevelOrder(TreeNode root) {
    LinkedList<TreeNode> queue = new LinkedList<>();
    queue.offer(root);
    while (!queue.isEmpty()) {
        TreeNode current = queue.poll();
        System.out.print(current.val + "\t");
        if (current.left != null) {
            queue.offer(current.left);
        }
        if (current.right != null) {
            queue.offer(current.right);
        }
    }
    System.out.println();
}