Non-decreasing Array LeetCode solution in C++

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class Solution {
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 public:
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  bool checkPossibility(vector<int> &nums) {
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    if (nums.size() <= 2) return true;
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    // creating a diff array(size=n-1) to store differences bw 2 consecutive
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    // ele.
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    vector<int> diff(nums.size() - 1);
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    int neg = 0;
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    for (int i = 0; i < nums.size() - 1; i++) {
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      diff[i] = nums[i + 1] - nums[i];
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      diff[i] < 0 ? neg++ : 0; // no of negative differences
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    }
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    if (neg > 1) return false;
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    for (int i = 0; i < diff.size(); i++) {
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      if (diff[i] < 0 and i != diff.size() - 1) {
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        if (i == 0)
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          // if(diff[i+1]>=0)
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          continue; // if the first diff is neg, the next is obviosly>=0 as
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                    // neg<=1, and this mountain can be resolved;
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        // else
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        // return false;
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        if ((nums[i - 1] <= nums[i + 1]) || (diff[i + 1]) >= abs(diff[i]))
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          continue; // the major mountain cases, lemme know below if want
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                    // explaination of these
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        else
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          return false;
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      }
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    }
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    return true;
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  }