二叉树递归遍历

递归三部曲

使用递归遍历二叉树，按照如下三步顺序：

• 确定递归函数的参数和返回值
• 确定终止条件
• 确定单层递归的逻辑

参考代码

前序递归遍历

class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> ret = new ArrayList<>();
        preorder(root, ret);
        return ret;
    }

    // 前序递归遍历
    public void preorder(TreeNode head, List<Integer> list) {
        if(head == null) {
            return;
        }
        list.add(head.val);
        preorder(head.left, list);
        preorder(head.right, list);
    }
}

中序递归遍历

class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> ans = new ArrayList<>();
        inorder(root, ans);
        return ans;
    }

    // 中序递归遍历
    public void inorder(TreeNode head, List<Integer> list) {
        if(head == null) {
            return;
        }
        inorder(head.left, list);
        list.add(head.val);
        inorder(head.right, list);
    }
}

后序递归遍历

class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> ans = new ArrayList<>();
        postorder(root, ans);
        return ans;
    }

    // 后序递归遍历
    public void postorder(TreeNode head, List<Integer> list) {
        if(head == null) {
            return;
        }
        postorder(head.left, list);
        postorder(head.right, list);
        list.add(head.val);
    }
}