Find a Value of a Mysterious Function Closest to Target LeetCode solution in Java

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class Solution {
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  int[] tree;
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  int[] arr;
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  int target;
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  int min;
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  public int closestToTarget(int[] arr, int target) {
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    int n = arr.length;
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    this.arr = arr;
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    this.target = target;
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    tree = new int[n << 2];
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    Arrays.fill(tree, (1 << 31) - 1); // initialize
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    min = Integer.MAX_VALUE;
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    for (int i = 0; i < n; i++) {
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      add(i, 0, n, 0);
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    }
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    return min;
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  }
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  private void add(int x, int l, int r, int n) {
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    if (l == r) {
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      tree[n] = arr[x];
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      min = Math.min(min, Math.abs(tree[n] - target));
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      return;
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    }
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    int mid = l + (r - l) / 2;
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    if (x <= mid) {
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      add(x, l, mid, 2 * n + 1);
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    } else {
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      add(x, mid + 1, r, 2 * n + 2);
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    }
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    tree[n] = (tree[2 * n + 1] & tree[2 * n + 2]); // & two subtrees
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    min = Math.min(min, Math.abs(tree[n] - target));
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  }
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}