Binary Tree Inorder Traversal

For example:
Given binary tree {1,#,2,3},

1
    \
     2
    /
   3

return [1,3,2].

二叉樹的中序遍歷，解題思路類同於前序遍歷。

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