144. 二叉树的前序遍历 94中序 145 后序

前序遍历

中左右
递归：

/**
 * Definition for a binary tree 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;
 *     }
 * }
 */
class Solution {
    private List<Integer> res = new ArrayList<>();
    public List<Integer> preorderTraversal(TreeNode root) {
        preOrder(root);
        return res;
    }

    public void preOrder(TreeNode root) {
        if (root == null)
            return ;
        res.add(root.val);
        preDfs(root.left);
        preDfs(root.right);
    }
}

中序遍历

左中右
递归：

class Solution {
    private List<Integer> res = new ArrayList<>();
    public List<Integer> inorderTraversal(TreeNode root) {
        inOrder(root);
        return res;
    }
    public void inOrder(TreeNode root) {
        if (root == null)
            return ;
        inOrder(root.left);
        res.add(root.val);
        inOrder(root.right);
    }
}

后序遍历

左右中
递归：

class Solution {
    private List<Integer> res = new ArrayList<>();
    public List<Integer> postorderTraversal(TreeNode root) {
        postOrder(root);
        return res;
    }

    public void postOrder(TreeNode root) {
        if (root == null)
            return ;
        postOrder(root.left);
        postOrder(root.right);
        res.add(root.val);
    }
}