sort-array-by-parity.cpp

// Time:  O(n)
// Space: O(1)

class Solution {
public:
    vector<int> sortArrayByParity(vector<int>& A) {
        for (int i = 0, j = 0; j < A.size(); ++j) {
            if (A[j] % 2 == 0) {
                swap(A[i++], A[j]);
            }
        }
        return A;
    }
};