## Facebook Interview Question for Software Engineers

Country: United States
Interview Type: Phone Interview

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Get the longest arithmetic progression:

``````public static int[] llac(int[] array) {
if(array.length == 0) return new int[]{};
int maxLen = 1, //len of longest sequence
end = array[0], //end number of sequence
step = 0;
Map<Integer,Map<Integer, Integer>> index_to_llac = new HashMap<>(); //Map<currentIndex, Map<difference, length>>

for(int current = 1; current < array.length; current++) {
index_to_llac.put(current, new HashMap<>());
for(int prev = 0; prev < current; prev++) {
int diff = array[current] - array[prev];
int length = 2;
if(index_to_llac.containsKey(prev) && index_to_llac.get(prev).containsKey(diff)) {
length = index_to_llac.get(prev).get(diff) + 1;
}
Map<Integer, Integer> diff_to_llac = index_to_llac.get(current);
//if this is a arithmetic progression with [diff] exists before current, add 1 to the length to include current in the progression
//otherwise, [prev, current] forms a progression of size 2
diff_to_llac.put(diff, length);

if(maxLen < length) {
maxLen = length;
end = array[current];
step = diff;
}
}
}

int[] sequence = new int[maxLen];
for(int i = maxLen - 1; i >= 0; i--) {
sequence[i] = end;
end -= step;
}
return sequence;
}``````

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Writing Code? Surround your code with {{{ and }}} to preserve whitespace.

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