chore: merge sort on An array, IN Place

That is: Space complexity is O(1), handy implement is O(n)
STD lib is O(nlogn)

daily/June6.cpp

@@ -22,4 +22,45 @@ class Solution {
     }
     return ans;
   }
+
+  /**
+   * Sorted Merge LCCI [E]
+   * 1. from back to begin
+   * Time: O(n), Space: O(1)
+   */
+  void merge(vector<int>& A, int m, vector<int>& B, int n) {
+    int k = m + n - 1;
+    int i = m - 1, j = n - 1;
+    while (k >= 0 && (i >= 0 || j >= 0)) {
+      if (i >= 0 && j >= 0 && A[i] >= B[j]) {
+        A[k] = A[i];
+        i--;
+        k--;
+      } else if (i >= 0 && j >= 0 && A[i] < B[j]) {
+        A[k] = B[j];
+        j--;
+        k--;
+      } else if (i >= 0 && j < 0) {
+        A[k] = A[i];
+        i--;
+        k--;
+      } else if (i < 0 && j >= 0) {
+        A[k] = A[j];
+        j--;
+        k--;
+      }
+    }
+  }
+
+  /**
+   * Use STD library of std::sort
+   * Time: Average: O(nlogn)
+   * Both works, but this is easy to implement, no much edge case
+   */
+  void mergeSTD(vector<int>& A, int m, vector<int>& B, int n) {
+    for (int i = 0; i < n; i++) {
+      A[m + i] = B[i];
+    }
+    sort(A.begin(), A.end());
+  }
 };