二叉树的前序、中序、后序遍历

/**
 * 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 {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<Integer>();
        preorder(root,res);
        return res;
    }
 
    public void preorder(TreeNode root, List arr) {
        if (root == null) return;
        arr.add(root.val);
        preorder(root.left,arr);
        preorder(root.right,arr);
    }
}

94. 二叉树的中序遍历

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

145. 二叉树的后序遍历

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