145. 二叉树的后序遍历

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {//后序遍历，左右根，也就是根右左然后逆序！！！（先序遍历是跟左右，则后续遍历时候入栈时先入左边再右边）最后的结果逆序输出一下就行！
    public List<Integer> postorderTraversal(TreeNode root) {
List<Integer>res=new ArrayList<Integer>();
        if(root==null)return res;
        Stack<TreeNode> stack=new Stack<TreeNode>();
        stack.push(root);
        while(!stack.isEmpty())
        {
            TreeNode n=stack.pop();
            res.add(n.val);
            if(n.left!=null)
            {
                stack.push(n.left);
            }
            if(n.right!=null)
            {
                stack.push(n.right);
            }

        }
        Collections.reverse(res);
        return res;
    }
}