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class Solution {
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  public int singleNumber(int[] nums) {
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    Stack numStack = new Stack();
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    Arrays.sort(nums);
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    for (var i = 0; i < nums.length; ++i) {
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      numStack.push(nums[i]);
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      if (i < nums.length - 1 && nums[++i] != (int) numStack.peek()) break;
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    }
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    return (int) numStack.pop();
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  }
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}