题解 | #二叉树的后序遍历#

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param root TreeNode类 
     * @return int整型一维数组
     */
    public int[] postorderTraversal (TreeNode root) {
        // write code here
        List<Integer> list = new ArrayList<>();
        Deque<TreeNode> stack = new LinkedList<>();
        TreeNode pre = null;
        while(true){
            while(root != null){
                stack.push(root);
                 root = root.left;
            }
            
            if(stack.isEmpty()) break;
            TreeNode peek = stack.peek();
            
            if(peek.right == null || pre == peek.right ) {
                list.add(peek.val);
                pre = stack.pop();
                peek = stack.peek();
            }else {
                root = peek.right;
            }
        }
        
        int[] ans = new int[list.size()];
        for(int i = 0; i < list.size(); i++) ans[i] = list.get(i);
        return ans;
    }
}