feat: solve missing number

Author: GangBean

missing-number/GangBean.java

@@ -0,0 +1,19 @@
+class Solution {
+    public int missingNumber(int[] nums) {
+        /**
+        1. understanding
+        - array nums, n distinct numbers in range [0, n]
+        - find missing number
+        2. strategy
+        - you can calculate the sum of range [0, n]: n(n+1)/2 ... (1)
+        - and the sum of nums ... (2)
+        - and then extract (2) from (1) = (missing value) what we want.
+        3. complexity
+        - time: O(N), N is the length of nums
+        - space: O(1)
+        */
+        int N = nums.length;
+        return N*(N+1)/2 - Arrays.stream(nums).sum();
+    }
+}
+