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class Solution {
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  public int[] findErrorNums(int[] nums) {
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    int N = nums.length, sum = N * (N + 1) / 2;
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    int[] ans = new int[2];
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    boolean[] seen = new boolean[N + 1];
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    for (int num : nums) {
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      sum -= num;
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      if (seen[num]) ans[0] = num;
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      seen[num] = true;
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    }
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    ans[1] = sum + ans[0];
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    return ans;
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  }
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}