LeetCode --- 94. 二叉树的中序遍历

• LeetCode链接：94. 二叉树的中序遍历

解法1 - 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<>();
    	preorder(list,root);
    	return list;
    }
    private void inorder(List<Integer> list, TreeNode node) {
    	if (node == null) return; 
    	list.add(node.val);
    	inorder(list, node.left);
    	inorder(list, node.right);
	}
}

解法2 - 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) {
        if (root == null) return new ArrayList<>();
        List<Integer> list = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<TreeNode>();
        while (root != null || !stack.isEmpty()) {
			if (root != null) {   
                stack.push(root);
				root = root.left;
			} else {
				TreeNode node = stack.pop();
				list.add(node.val);
				root = node.right;
			}
        }
		return list;
    }
}