Binary Tree Cameras LeetCode solution in JavaScript

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var minCameraCover = function (root) {
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  let cam = 0;
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  // 0 --> No covered
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  // 1 --> covered by camera
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  // 2 --> has camera
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  function dfs(root) {
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    if (root === null) return 1;
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    const left = dfs(root.left);
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    const right = dfs(root.right);
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    if (left === 0 || right === 0) {
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      // child required a camera to covered
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      cam++;
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      return 2;
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    } else if (left === 2 || right === 2) {
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      // child has camera so i am covered
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      return 1;
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    } else {
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      // child is covered but don't have camera,So i want camera
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      return 0;
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    }
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  }
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  const ans = dfs(root);
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  if (ans === 0) ++cam;
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  return cam;
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};