二叉树的遍历

定义Node

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

    TreeNode() {
    }

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

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

前序遍历

List<Integer> integers = new ArrayList<>();

public List<Integer> preorderTraversal(TreeNode root) {
    if (root == null) return integers;
    integers.add(root.val);
    if (root.left != null) {
        preorderTraversal(root.left);
    }
    if (root.right != null) {
        preorderTraversal(root.right);
    }
    return integers;
}

中序遍历

List<Integer> integers = new ArrayList<>();

public List<Integer> inorderTraversal(TreeNode root) {
    if (root == null) return integers;
    if (root.left != null) {
        inorderTraversal(root.left);
    }
    integers.add(root.val);
    if (root.right != null) {
        inorderTraversal(root.right);
    }
    return integers;
}

后序遍历

List<Integer> integers = new ArrayList<>();

public List<Integer> postorderTraversal(TreeNode root) {
    if (root == null) return integers;
    if (root.left != null) {
        postorderTraversal(root.left);
    }
    if (root.right != null) {
        postorderTraversal(root.right);
    }
    integers.add(root.val);
    return integers;
}