Find Max Difference in Array - Need Algorithm Solution Optimization [duplicate]

Question

I practised solving an algo on HackerRank - Max Difference.

Here's the problem given:

You are given an array with n elements: d[ 0 ], d[ 1 ], ..., d[n-1]. Calculate the sum(S) of all contiguous sub-array's max difference.

Formally:

S = sum{max{d[l,...,r]} - min{d[l, ..., r}},∀ 0 <= l <= r < n 

Input format:

n
d[0] d[1] ... d[n-1]

Output format:

S

Sample Input:

4
1 3 2 4

Sample Output:

12

Explanation:

l = 0; r = 0;
array: [1]
sum = max([1]) - min([1]) = 0

l = 0; r = 1;
array: [1,3]
sum = max([1,3]) - min([1,3]) = 3 - 1 = 2

l = 0; r = 2;
array: [1,3,2]
sum = max([1,3,2]) - min([1,3,2]) = 3 - 1 = 2

l = 0;r = 3;
array: [1,3,2,4]
sum = max([1,3,2,4]) - min([1,3,2,4]) = 4 - 1 = 3

l = 1; r = 1 will result in zero

l = 1; r = 2;
array: [3,2]
sum = max([3,2]) - min([3,2]) = 3 - 2 = 1;

l = 1; r = 3;
array: [3,2,4]
sum = max ([3,2,4]) - min([3,2,4]) = 4 - 2 = 2;

l = 2; r = 2; will result in zero

l = 2; r = 3;
array:[2,4]
sum = max([2,4]) - min([2,4]) = 4 -2 = 2;

l = 3; r = 3 will result in zero;

Total sum = 12

Here's my solution:

-(NSNumber*)sum:(NSArray*) arr {

int diff = 0;
int curr_sum = diff;
int max_sum = curr_sum;

for(int i=0; i<arr.count; i++)
{
    for(int j=i; j<=arr.count; j++) {
        // Calculate current diff
        if (!(j-i > 1)) {
            continue;
        }

        NSArray *array = [arr subarrayWithRange:NSMakeRange(i, j-i)];
        if (!array.count || array.count == 1) {
            continue;
        }
        int xmax = -32000;
        int xmin = 32000;
        for (NSNumber *num in array) {
            int x = num.intValue;
            if (x < xmin) xmin = x;
            if (x > xmax) xmax = x;
        }
        diff = xmax-xmin;

        // Calculate current sum
        if (curr_sum > 0)
           curr_sum += diff;
        else
           curr_sum = diff;

        // Update max sum, if needed
        if (curr_sum > max_sum)
           max_sum = curr_sum;   
    }
}

return @(max_sum);

}

There were totally 10 test cases. The above solution passed first 5 test cases, but didn't get passed through the other 5, which were failed due to time out (>=2s).

"Here's the Status: Terminated due to timeout".

Please help me on how this code can be further optimised.

Thanks!

Answer 1

Already there was an answer in Python. Here's the Objective C version from me:

@interface Stack : NSObject {
    NSMutableArray* m_array;
    int count;
}
- (void)push:(id)anObject;
- (id)pop;
- (id)prev_prev;
- (void)clear;
@property (nonatomic, readonly) NSMutableArray* m_array;
@property (nonatomic, readonly) int count;
@end

@implementation Stack
@synthesize m_array, count;
- (id)init
{
    if( self=[super init] )
    {
        m_array = [[NSMutableArray alloc] init];
        count = 0;
    }
    return self;
}
- (void)push:(id)anObject
{
    [m_array addObject:anObject];
    count = m_array.count;
}
- (id)pop
{
    id obj = nil;
    if(m_array.count > 0)
    {
        obj = [m_array lastObject];
        [m_array removeLastObject];
        count = m_array.count;
    }
    return obj;
}
- (id)prev_prev
{
    id obj = nil;
    if(m_array.count > 0)
    {
        obj = [m_array lastObject];
    }
    return obj;
}
- (void)clear
{
    [m_array removeAllObjects];
    count = 0;
}
@end

@interface SolutionClass:NSObject
/* method declaration */
-(NSNumber*)findDiff:(NSArray*) arr;
@end


@implementation SolutionClass
-(NSNumber*)findDiff:(NSArray*) arr {
    NSNumber *maxims = [self sum:arr negative:NO];
    NSNumber *minims = [self sum:arr negative:YES];
    NSNumber *diff = @(maxims.longLongValue+minims.longLongValue);
    NSLog(@"diff: %@", diff);
    return diff;
}

-(NSNumber*)sum:(NSArray*) arr negative:(BOOL)negate {
    Stack *stack = [Stack new];
    [stack push:@{@(-1): [NSNull null]}];
    long long sum = 0;

    for(int i=0; i<arr.count; i++) {
        NSNumber *num = arr[i];
        if (negate) {
            num = @(-num.longLongValue);
        }

        NSDictionary *prev = stack.m_array.lastObject;
        NSNumber *prev_i = (NSNumber*)prev.allKeys[0];
        NSNumber *prev_x = (NSNumber*)prev.allValues[0];

        if ([self isNumber:prev_x]) {
            while (num.longLongValue > prev_x.longLongValue) {
                prev_i = (NSNumber*)prev.allKeys[0];
                prev_x = (NSNumber*)prev.allValues[0];
                prev = [stack pop];
                NSDictionary *prev_prev_Dict = [stack prev_prev];
                NSNumber *prev_prev_i = (NSNumber*)prev_prev_Dict.allKeys[0];

                sum += prev_x.longLongValue * (i-prev_i.longLongValue) * (prev_i.longLongValue - prev_prev_i.longLongValue);

                prev = stack.m_array.lastObject;
                prev_x = (NSNumber*)prev.allValues[0];
                if (![self isNumber:prev_x]) {
                    break;
                }

            }
        }
        [stack push:@{@(i): num}];
    }
    NSLog(@"Middle: sum: %lld", sum);

    while (stack.count > 1) {
        NSDictionary *prev = [stack pop];
        NSDictionary *prev_prev_Dict = [stack prev_prev];

        NSNumber *prev_i = (NSNumber*)prev.allKeys[0];
        NSNumber *prev_x = (NSNumber*)prev.allValues[0];
        NSNumber *prev_prev_i = (NSNumber*)prev_prev_Dict.allKeys[0];

        sum += prev_x.longLongValue * (arr.count-prev_i.longLongValue) * (prev_i.longLongValue - prev_prev_i.longLongValue);

        prev = stack.m_array.lastObject;
        prev_x = (NSNumber*)prev.allValues[0];
        if (![self isNumber:prev_x]) {
            break;
        }
    }
    NSLog(@"End: sum: %lld", sum);

    return @(sum);
}

-(BOOL)isNumber:(id)obj {
    if ([obj isKindOfClass:[NSNumber class]]) {
        return 1;
    }
    return 0;
}
@end

The above solution works well for 7 test cases, but fails for the other 3 saying this: "Status: Wrong Answer". Hoping to find a fix for that too.

EDIT:

Have updated the WORKING code that passed all the test cases. Wrong data types were used before.