"Wrong Answer" Google Codejam 2020 Question-3 Parenting Partnering Returns

I have tried question so many test cases on my own too and I’m getting right answers but whenever I submit it is giving me Wrong Answer. Please Help me with my code. I am using bruteforce approch here.

Please suggest any test case that would help solve my problem.
Here is my code with proper comments so please have a look at it.

       /*
		Input are taken in these two arrays respectively
		
		int[] start=new int[n];
		int[] end=new int[n];
                                                               */
		
		int[][] j=new int[n][2]; //arrays to store the start and end time at j[i][0] ans j[i][1] respectively for the timimgs of activity on which J will be busy
		int[][] c=new int[n][2]; // similar array for c
		for(int i=0;i<n;i++) {
			for(int k=0;k<2;k++) {  //initialising array to -1 because 0 initialization would lead to wrong overlapping
				j[i][k]=-1;
				c[i][k]=-1;
			}
		}
		
		int f=1, zj=0, zc=0;  // f is flag variable ans zj and zc are used to iterate over  j and c arrays.
		String ans="", im="IMPOSSIBLE";
		for(int i=0;i<n;i++) {
			for(int k=0;k<n;k++) {
				if(isOverlapping(start[i], end[i], j[k][0], j[k][1])) {
					f=0;  // if current timing is overlapping with any previous of J than J will not be added.
					break;
				}else f=1;
			}
			if(f==0) {
				for(int k=0;k<n;k++) {
					if(!isOverlapping(start[i], end[i], c[k][0], c[k][1])) {
						f=2;  
					}else {
						f=0; // If Ccurrent timing of C also overlaps with any other previous timings of c that it is IMPOSSIBLE
						break;
					}
				}
			}
			if(f==1) {  // If J is free 
				j[zj][0]=start[i];
				j[zj][1]=end[i];
				zj++;
				ans+='J';
			}else if(f==2) { // If C is free
				c[zc][0]=start[i];
				c[zc][1]=end[i];
				zc++;
				ans+='C';
				f=1;
			}else { //If both are busy
				ans=im;
				break;
			}
		}

The right answer.