645. Set Mismatch 

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class Solution {
    public int[] findErrorNums(int[] nums) {
        int[] res = new int[2];

        for (int i = 0; i < nums.length; i++) {
            int num = Math.abs(nums[i]);
            if (nums[num - 1] > 0) {
                nums[num - 1] *= -1;
            } else {
                // 如果已经小于 0 了，说明数字重复，注意此处应该取 num
                res[0] = num;
            }
        }

        for (int i = 0; i < nums.length; i++) {
            if (nums[i] > 0) {
                // 如果没有被翻转，说明该索引本该存储的数字丢失
                res[1] = i + 1;
            }
        }

        return res;
    }
}
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class Solution {
    public int[] findErrorNums(int[] nums) {
        for (int i = 0; i < nums.length; i++) {
            while (nums[i] != nums[nums[i] - 1]) {
                swap(nums, i, nums[i] - 1);
            }
        }

        for (int i = 0; i < nums.length; i++) {
            if (nums[i] != i + 1) {
                return new int[]{nums[i], i + 1}; // 注意按题目顺序返回
            }
        }

        return new int[0];
    }

    private void swap(int[] nums, int i, int j) {
        int tmp = nums[i];
        nums[i] = nums[j];
        nums[j] = tmp;
    }
}

References

645. Set Mismatch