Binary Tree Cameras LeetCode solution in Java

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/**
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 * Definition for a binary tree node. public class TreeNode { int val; TreeNode left; TreeNode
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 * right; TreeNode() {} TreeNode(int val) { this.val = val; } TreeNode(int val, TreeNode left,
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 * TreeNode right) { this.val = val; this.left = left; this.right = right; } }
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 */
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class Solution {
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  private int count = 0;
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  public int minCameraCover(TreeNode root) {
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    if (helper(root) == -1) count++;
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    return count;
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  }
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  // post order
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  // 0 - have camera
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  // 1 - covered
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  // -1 - not covered
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  public int helper(TreeNode root) {
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    if (root == null) return 1;
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    int left = helper(root.left);
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    int right = helper(root.right);
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    if (left == -1 || right == -1) {
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      count++;
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      return 0;
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    }
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    if (left == 0 || right == 0) return 1;
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    return -1;
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  }
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}