3354. Make Array Elements Equal to Zero

You are given an integer array nums.

Start by selecting a starting position curr such that nums[curr] == 0, and choose a movement direction of either left or right.

After that, you repeat the following process:

• If curr is out of the range [0, n - 1], this process ends.

• If nums[curr] == 0, move in the current direction by incrementing curr if you are moving right, or decrementing curr if you are moving left.

• Else if nums[curr] > 0:

  • Decrement nums[curr] by 1.
  • Reverse your movement direction (left becomes right and vice versa).
  • Take a step in your new direction.

A selection of the initial position curr and movement direction is considered valid if every element in nums becomes 0 by the end of the process.

Return the number of possible valid selections.

class Solution {
public:
    int countValidSelections(vector<int>& nums) {
        int ri = accumulate(begin(nums), end(nums), 0), le = 0, res = 0;
        for(int i = 0; i < nums.size(); i++) {
            if(!nums[i]) {
                if(le == ri) res += 2;
                else if(abs(le-ri) == 1) res += 1;
            }
            le += nums[i], ri -= nums[i];
            if(le > ri + 1) return res;
        }
        return res;
    }
};