求樹的中序遍歷，遞迴很簡單，迴圈我用棧來實現，再用hashmap或者改變變數的值來記錄一下。遞迴0ms，beats 100%，迴圈：改變值，1ms，beats 59.19%，用hashmap 2ms= =

只放迴圈的吧。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    
    LinkedList
 
<Integer> ans = new LinkedList<>();
     
    public List<Integer> inorderTraversal(TreeNode root) {
        
        Stack st = new Stack();
        HashMap map = new HashMap();
        TreeNode temp;
        if(root == null) return ans;
        st.add(root);
        while(st.isEmpty() != true
 
){
            
            temp = (TreeNode)st.peek();
            if(temp.left != null){
                
                st.add(temp.left);
                temp.left = null;
                continue;
            }
            ans.add(temp.val);
            st.pop();
            if(temp.right != null
 
) {
                st.add(temp.right);
                temp.right = null;
            }
            
        }
        return ans;
    }
}