Added 2180

Author: trung-hn

Readme.md

@@ -651,6 +651,7 @@
 | 2140 | [Solving Questions With Brainpower](src/2140.solving-questions-with-brainpower.py)                                                                                   | Medium | O(N)                  | O(N)           | Array, Dynamic Programming                                                                           |                                         |           |
 | 2176 | [Count Equal and Divisible Pairs in an Array](src/2176.count-equal-and-divisible-pairs-in-an-array.py)                                                               | Easy   | O(N^2)                | O(1)           | Array                                                                                                |                                         |           |
 | 2177 | [Count Equal and Divisible Pairs in an Array](src/2177.find-three-consecutive-integers-that-sum-to-a-given-number.py)                                                | Medium | O(1)                  | O(1)           | Math, Binary Search                                                                                  |                                         |           |
+| 2180 | [Count Integers With Even Digit Sum](src/2180.count-integers-with-even-digit-sum.py)                                                                                 | Easy   | O(1)                  | O(1)           | Math, Simulation                                                                                     |                                         |           |
 | 2185 | [Counting Words With a Given Prefix](src/2185.counting-words-with-a-given-prefix.py)                                                                                 | Easy   | O(N)                  | O(1)           | Array, String                                                                                        |                                         |           |
 | 2186 | [Minimum Number of Steps to Make Two Strings Anagram II](src/2186.minimum-number-of-steps-to-make-two-strings-anagram-ii.py)                                         | Medium | O(N)                  | O(N)           | Hash Table, String, Counting                                                                         |                                         |           |
 | 2190 | [Most Frequent Number Following Key In an Array](src/2190.most-frequent-number-following-key-in-an-array.py)                                                         | Easy   | O(N)                  | O(N)           | Array, Hash Table, Counting                                                                          |                                         |           |

src/2180.count-integers-with-even-digit-sum.py

@@ -0,0 +1,25 @@
+#
+# @lc app=leetcode id=2180 lang=python3
+#
+# [2180] Count Integers With Even Digit Sum
+#
+
+# @lc code=start
+# TAGS: Math, Simulation
+class Solution:
+    def countEven(self, num: int) -> int:
+        cnt = 0
+        for i in range(1, num + 1):
+            cnt += sum(int(c) for c in str(i)) % 2 == 0
+        return cnt
+
+    # Math
+    # For a num with even sum of its digits, count of Integers With Even Digit Sum less than or equal to num is num/2
+    # For a num with odd sum of its digits, count of Integers With Even Digit Sum less than or equal to num is (num-1)/2
+    # Time and Space: O(1)
+    def countEven1(self, num: int) -> int:
+        if sum(int(c) for c in str(num)) % 2:
+            return (num - 1) // 2
+        return num // 2
+
+# @lc code=end