docs: add complexity analysis

Author: HiGeuni

3sum/higeuni.js

@@ -1,6 +1,11 @@
 // complexity
 // time: O(n^2)
+// - sort: O(n log n)
+// - for loop: O(n)
+// - while loop: O(n)
 // space: O(n)
+// - sortedNums: O(n)
+// - else : O(1)
 
 var threeSum = function(nums) {
   const sortedNums = nums.sort((a, b) => a - b)

climbing-stairs/higeuni.js

@@ -1,6 +1,8 @@
 // complexity
 // time: O(n)
+// - for loop: O(n)
 // space: O(1)
+// - n에 관계없이 상수개의 변수만 사용되므로 O(1)
 
 var climbStairs = function(n) {
   let num1 = 1;

valid-anagram/higeuni.js

@@ -1,6 +1,12 @@
 // complexity
 // time: O(n log n)
+// - sort: O(n log n)
+// - split: O(n)
+// - join: O(n)
 // space: O(n)
+// - sortedS: O(n)
+// - sortedT: O(n)
+// - else : O(1)
 
 var isAnagram = function(s, t) {
   return s.split('').sort().join('') === t.split('').sort().join('')