递归的代码是以前数据结构书上常见的：

    public ArrayList<Integer> inorderTraversal(ConstructBinaryTreefromPostorderandInorderTraversal.TreeNode root) {ArrayList<Integer> res = new ArrayList<>();dfs(res, root);return res;}private void dfs(List<Integer> res, ConstructBinaryTreefromPostorderandInorderTraversal.TreeNode node) {if (node == null) return;dfs(res, node.left);res.add(node.val);dfs(res, node.right);}
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非递归用stack模拟中序遍历，要理解啊，不能死记硬背。注意while条件和while里面的if。

public class Solution {public ArrayList<Integer> inorderTraversal(TreeNode root) {ArrayList<Integer> res = new ArrayList<>();LinkedList<TreeNode> stack = new LinkedList<>();//注意while条件是或while (root != null || !stack.isEmpty()){if (root!=null){stack.push(root);root = root.left;}else {root = stack.pop();res.add(root.val);root = root.right;}}return res;}
}
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MAR 25TH 这题今天做98. Validate Binary Search Tree 的时候又来复习了一遍，又忘的差不多了。。记得当时还在考虑为什么while里面要用||而不是&&。

这题还可以这样写：

public List<Integer> inorderTraversal(TreeNode root) {List<Integer> list = new ArrayList<>();if(root == null) return list;Stack<TreeNode> stack = new Stack<>();while(root != null || !stack.empty()){while(root != null){stack.push(root);root = root.left;}root = stack.pop();list.add(root.val);root = root.right;}return list;
}
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JULY 29 REVIEW

又看了一遍迭代的方法，仍然写不出。。上面那个代码，两个while循环其实挺清晰的，但是root = root.right那边还是挺难想到的。还有就是外层的while，两种情况；1是root!= null的情况，这种比较好考虑，就是首次进入的时候；2是root==null的情况，statck不为空，这种就是dfs到栈最底的情况。

转载于:https://juejin.im/post/5a3132fef265da432d281a4b

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