94.二叉树的中序遍历

//递归
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    List<Integer> list = new ArrayList<Integer>();
    public List<Integer> inorderTraversal(TreeNode root) {
        if(root!=null) {
            inorderTraversal(root.left);
            list.add(root.val);
            inorderTraversal(root.right);
        }
        return list;
    }
}

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