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128-Longest-consecutive-sequence.js
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128-Longest-consecutive-sequence.js
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//////////////////////////////////////////////////////////////////////////////
// Linear Search With A Hash Map
// Time: O(n)
// Space: O(n)
// This solution only makes one pass over the `nums` array and is the highest
// performing solution.
//////////////////////////////////////////////////////////////////////////////
/**
* @param {number[]} nums
* @return {number}
*/
function longestConsecutive(nums) {
if (!nums.length) {
return 0;
}
const map = Object.create(null);
let max = 0;
for (const num of nums) {
if (num in map) {
continue;
}
const prev = num - 1;
const next = num + 1;
let len = 1;
if (prev in map) {
if (next in map) {
len += map[prev] + map[next];
map[prev - map[prev] + 1] = len;
map[next + map[next] - 1] = len;
} else {
len += map[prev];
++map[prev - map[prev] + 1];
}
} else if (next in map) {
len += map[next];
++map[next + map[next] - 1];
}
map[num] = len;
max = Math.max(max, len);
}
return max;
}
//////////////////////////////////////////////////////////////////////////////
// Linear Search With A Hash Set
// Time: O(n)
// Space: O(n)
// This solution does three passes over the `nums` array. A first pass to
// setup the hash set. A second pass to find the numbers that mark the
// beginning of a sequence. A third pass to calculate the length of each
// sequence. The nested `while` loop does not cause quadratic calculations as
// it is only initiated on the first number of each sequence.
//////////////////////////////////////////////////////////////////////////////
/**
* @param {number[]} nums
* @return {number}
*/
function longestConsecutive(nums) {
let len = nums.length;
if (!len) {
return 0;
}
const set = new Set(nums);
let max = 0;
for (let i = 0; i < nums.length; i++) {
const num = nums[i];
if (set.has(num - 1)) {
continue;
}
let currentMax = 1;
while (set.has(num + currentMax)) {
currentMax++;
}
if (currentMax > max) {
max = currentMax;
}
if(max > len/2){
break;
}
}
return max;
}