递归实现

/**
 * 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> inorderTraversal(TreeNode root) {
        List<Integer> res=new ArrayList<Integer>();
        accessTree(root,res);
        return res;
    }

    public void accessTree(TreeNode root, List<Integer> res){
        if(root==null){
            return;
        }
        accessTree(root.left,res);
        res.add(root.val);
        accessTree(root.right,res);
    }
}

栈实现

/**
 * 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> inorderTraversal(TreeNode root) {
        List<Integer> res=new ArrayList<Integer>();
        Deque<TreeNode> stack = new LinkedList<TreeNode>();
       while(root!=null||!stack.isEmpty()){
            while(root!=null){
            stack.push(root);
            root=root.left;
        }
            root=stack.pop();
            res.add(root.val);
            root=root.right;
       }
        return res;
    }
}