LeetCode --- 145. 二叉树的后序遍历

• LeetCode链接：145. 二叉树的后序遍历

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> postorderTraversal(TreeNode root) {
    	List<Integer> list = new ArrayList<Integer>();
    	postorderTraversal(root, list);

    	return list;
    }
    private void postorderTraversal(TreeNode root, List<Integer> list) {
		if (root == null) return;
		postorderTraversal(root.left, list);
		postorderTraversal(root.right, list);
		list.add(root.val);
	}
}

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> postorderTraversal(TreeNode root) {
        if (root == null) return new LinkedList<Integer>();
        LinkedList<Integer> list = new LinkedList<Integer>();
        Stack<TreeNode> stack = new Stack<TreeNode>();
        while (root != null || !stack.isEmpty()) {
			if (root != null) {
				list.addFirst(root.val);
				stack.push(root);
				root = root.right;
			} else {
				TreeNode node = stack.pop();
				root = node.left;
			}
		}
        return list;
	}
}