二叉树的先中后序遍历

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var preorderTraversal = function(root) {
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  const res = []
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  const dfs = (root) => {
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    if(root === null) return
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    res.push(root.val)
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    dfs(root.left)
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    dfs(root.right)
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  }
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  dfs(root)
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  return res
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};

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var preorderTraversal = function(root) {
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  if (!root) return;
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  const res = []
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  const stack = [root];
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  while (stack.length > 0) {
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    const node = stack.pop();
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    res.push(node.val)
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    // 注意：因为栈是LIFO，所以先压入右子节点，再压入左子节点
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    if (node.right) stack.push(node.right);
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    if (node.left) stack.push(node.left);
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  }
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  return res
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};

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var inorderTraversal = function(root) {
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  const res = []
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  const dfs = (root) => {
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    if(root === null) return
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    dfs(root.left)
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    res.push(root.val)
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    dfs(root.right)
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  }
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  dfs(root)
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  return res
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};

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var postorderTraversal = function(root) {
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  const res = []
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  const dfs = (root) => {
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    if(root === null) return
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    dfs(root.left)
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    dfs(root.right)
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    res.push(root.val)
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  }
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  dfs(root)
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  return res
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};