//非遞歸
//根 左 右
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        //用數(shù)組來存儲(chǔ)前序遍歷結(jié)果
        List<Integer> list = new ArrayList<>();
        if(root==null)  return list;
        
        //創(chuàng)建一個(gè)棧
        Stack<TreeNode> st = new Stack<>();
        //cur指向根節(jié)點(diǎn)
        TreeNode cur = root;
        
        while(!st.isEmpty()||cur!=null){
        
            //一邊向左遍歷，一邊將遍歷到的節(jié)點(diǎn)入棧，節(jié)點(diǎn)值入數(shù)組
            while(cur!=null){
                list.add(cur.val);
                st.push(cur);	//根
                cur=cur.left;	//左
            }
            
            //指針指向棧頂節(jié)點(diǎn)（即上一個(gè)節(jié)點(diǎn)），并將棧頂節(jié)點(diǎn)出棧
            cur = st.pop();
            //指針指向該節(jié)點(diǎn)的右子樹，開始下一輪的遍歷
            cur = cur.right;	//右
        }
        return list;
    }
}

//非遞歸
//左 根 右
class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        
      List<Integer> list = new ArrayList<>();
      if(root==null)    return list;
        
      Stack<TreeNode> st = new Stack<>();
       TreeNode cur = root;
      while(!st.isEmpty()||cur!=null){
      //一邊向左遍歷，一邊將遍歷到的節(jié)點(diǎn)入棧
          while(cur!=null){
              st.push(cur);
              cur = cur.left;	//左
          }
          cur = st.pop();
          //存儲(chǔ)遍歷結(jié)果
          list.add(cur.val);	//根
          cur = cur.right;		//右
      }
      return list;
    }
}

//非遞歸
//左 右 根
class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
     List<Integer> list = new ArrayList<>();
     if(root==null) return list;
     TreeNode cur = root;
     
     //關(guān)鍵在于定義一個(gè)cur的前驅(qū)節(jié)點(diǎn)
     TreeNode pre = null;
     Stack<TreeNode> st = new Stack<>();
     while(cur!=null||!st.isEmpty()){
     
     	//一邊向左遍歷，一邊將遍歷到的節(jié)點(diǎn)入棧
         while(cur!=null){
             st.push(cur);
            cur = cur.left;
         }
         cur = st.pop();
         //若該節(jié)點(diǎn)的右節(jié)點(diǎn)為空，或者右節(jié)點(diǎn)被遍歷過，數(shù)組才能存儲(chǔ)該節(jié)點(diǎn)的數(shù)值（也就是我們最后才遍歷的根）
         if(cur.right==null||cur.right==pre){
             list.add(cur.val);
             pre = cur;
             cur=null;
         }else{//如果不滿足，說明該節(jié)點(diǎn)的右節(jié)點(diǎn)還沒有被遍歷過，那么接著向右子節(jié)點(diǎn)遍歷
             st.push(cur);
             cur=cur.right;
         }
     }
     return list;
    }
}