94. Binary Tree Inorder Traversal

递归法： 

package auguest;

import java.util.ArrayList;
import java.util.List;

class TreeNode{
	
	int val;
	TreeNode left;
	TreeNode right;
	TreeNode(int val)
	{
		this.val=val;
	}
	
}

public class BinaryTreeInorderTraversal {

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

非递归法：

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