144. Binary Tree Preorder Traversal.ipynb

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    "Given a binary tree, return the preorder traversal of its nodes' values.\n",
    "\n",
    "For example:\n",
    "Given binary tree {1,#,2,3},\n",
    "   1\n",
    "    \\\n",
    "     2\n",
    "    /\n",
    "   3\n",
    "return [1,2,3].\n",
    "\n",
    "Note: Recursive solution is trivial, could you do it iteratively?"
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   "source": [
    "class TreeNode(object):\n",
    "    def __init__(self, x):\n",
    "        self.val = x\n",
    "        self.left = None\n",
    "        self.right = None"
   ]
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    "class Solution(object):\n",
    "    def preorderTraversal(self, root):\n",
    "        \"\"\"\n",
    "        :type root: TreeNode\n",
    "        :rtype: List[int]\n",
    "        \"\"\"\n",
    "        self.res=[]\n",
    "        self.helper(root)\n",
    "        return self.res\n",
    "    def helper(self,root):\n",
    "        if not root:\n",
    "            return\n",
    "        self.res.append(root.val)\n",
    "        self.helper(root.left)\n",
    "        self.helper(root.right)"
   ]
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    "t1=TreeNode(2)\n",
    "t2=TreeNode(0)\n",
    "t3=TreeNode(3)\n",
    "t1.left=t2\n",
    "t1.right=t3\n",
    "t4=TreeNode(4)\n",
    "t5=TreeNode(5)\n",
    "t6=TreeNode(6)\n",
    "t2.left=t4\n",
    "t2.right=t5\n",
    "t4.left=t6\n",
    "s=Solution()"
   ]
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    {
     "data": {
      "text/plain": [
       "[2, 0, 4, 6, 5, 3]"
      ]
     },
     "execution_count": 10,
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   "source": [
    "s.preorderTraversal(t1)"
   ]
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   "source": [
    "class Solution(object):\n",
    "    def preorderTraversal(self, root):\n",
    "        \"\"\"\n",
    "        :type root: TreeNode\n",
    "        :rtype: List[int]\n",
    "        \"\"\"\n",
    "        if not root:\n",
    "            return []\n",
    "        stack=[root]\n",
    "        res=[]\n",
    "        while stack:\n",
    "            node=stack.pop()\n",
    "            res.append(node.val)\n",
    "            if node.right:\n",
    "                stack.append(node.right)\n",
    "            if node.left:\n",
    "                stack.append(node.left)\n",
    "        return res"
   ]
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