LeetCode 754. Reach a Number

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class Solution {
public:
    int reachNumber(int target) {
        target = abs(target);
        int k = 0, sum = 0;
        while (sum < target || (sum - target) & 1)
            sum += ++k;
        return k;
    }
};
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class Solution {
public:
    int reachNumber(int target) {
        target = abs(target);
        long n = ceil((-1.0 + sqrt(1 + 8.0 * target)) / 2);
        long sum = n * (n + 1) / 2;
        if (sum == target) return n;
        long diff = sum - target;
        if ((diff & 1) == 0) return n;
        return n + ((n & 1) ? 2 : 1);
    }
};