PassingCars.java

package PassingCars;

class Solution {
    public int solution(int[] A) {
        // write your code in Java SE 8
        
        // note: find number of pairs (P, Q)
        // where P < Q (important)
        // try to use "one pass" solution (low time complexity)
        
        int num_east = 0; // initial
        int num_pass = 0; // initial
        
        for(int i=0; i<A.length; i++){
            if(A[i] ==0){ // to east
                num_east++;
            }
            if(A[i] ==1){ // to west
                num_pass = num_pass + num_east;
            }
            // note: just look back "num_east" 
            //       that will be the number of cars can be paired 
            //       (with the current car)
        }
        
        // note 1: can use "_" for a big value
        // note 2: "num_pass < 0" is for the "overflow" cases
        //         when "overflow" occurs, the value will "< 0" (important)
        if(num_pass > 1_000_000_000 || num_pass < 0) 
            return -1;
        else
            return num_pass;       
    }
}