leetcode 145. 二叉树的后序遍历

后序遍历：左右中，和前序遍历（中左右）只差了一点，只需调整结果顺序和左右节点入栈顺序即可。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> ans=new ArrayList<>();
        Deque<TreeNode> stack=new ArrayDeque<>();
        if(root==null){
            return ans;
        }
        stack.push(root);
        while(!stack.isEmpty()){
            TreeNode node=stack.poll();
            ans.add(node.val);
            if(node.left!=null){
                stack.push(node.left);
            }
            if(node.right!=null){
                stack.push(node.right);
            }
        }
        Collections.reverse(ans);
        return ans;
    }
}