upload 1 answer, Aug 24th

Author: stevenlordiam

Medium/MissingNumber.java

@@ -0,0 +1,39 @@
+/*
+Given an array containing n distinct numbers taken from 0, 1, 2, ..., n, find the one that is missing from the array.
+
+For example,
+Given nums = [0, 1, 3] return 2.
+
+Note:
+Your algorithm should run in linear runtime complexity. Could you implement it using only constant extra space complexity?
+*/
+
+// version 1, add up all numbers, O(N) time, O(1) space
+public class MissingNumber {
+    public int missingNumber(int[] nums) {
+        int sum = 0;
+        for(int num: nums)
+            sum += num;
+        return (nums.length * (nums.length + 1) )/ 2 - sum;
+    }
+}
+
+// version 2, XOR, O(1) space
+// n^n = 0 and n^0 = n
+public class MissingNumber {
+	public int missingNumber(int[] nums) {
+	    int result = 0;
+	    for (int i = 1; i <= nums.length; i++) {
+	        result ^= nums[i - 1];
+	        result ^= i;
+	    }
+	    return result;
+	}
+}
+
+/*
+Reference:
+https://leetcode.com/discuss/53778/java-solution-o-1-space-and-o-n-in-time
+https://leetcode.com/discuss/53794/simple-java-solution-with-explanation-time-o-n-space-o-1
+https://leetcode.com/discuss/53871/java-simplest-solution-o-1-space
+*/