LintCode-68.二叉树的后序遍历

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/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
public class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: Postorder in ArrayList which contains node values.
     */
    public ArrayList<Integer> postorderTraversal(TreeNode root) {
        // write your code here
        ArrayList<Integer> a = new ArrayList<Integer>();
        postOrder(a, root);
        return a;
    }
        
    private void postOrder(ArrayList<Integer> a, TreeNode root){
        if(root == null) return;
        postOrder(a, root.left);
        postOrder(a, root.right);
        a.add(root.val);
    }
}