题目：牛客网

解题思路：

非递归借用栈  

非递归： 

import java.util.ArrayList;
import java.util.Stack;
/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ArrayList<Integer> inorderTraversal(TreeNode root) {
		ArrayList<Integer> res = new ArrayList<Integer>();
		Stack<TreeNode> stack = new Stack<TreeNode>();
        if(null == root)
        	return res;
        TreeNode p = root;
        while(!stack.isEmpty()|| p!=null){
        	while(p != null){
            	stack.add(p);
            	p = p.left;
            }
        	TreeNode current = stack.pop();
        	res.add(current.val);
        	p = current.right;
        }
        return res;
    }
}

 递归：

 

import java.util.ArrayList;
/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ArrayList<Integer> inorderTraversal(TreeNode root) {
		ArrayList<Integer> res = new ArrayList<Integer>();
		if(null == root)
			return res;
		if(null != root.left){
			res.addAll(inorderTraversal(root.left));
		}
		res.add(root.val);
		if(null != root.right){
			res.addAll(inorderTraversal(root.right));
		}
		return res;
    }
}