diff --git "a/leetcode3/\354\235\264\354\247\204\355\235\254/315. Count of Smaller Numbers After Self.java" "b/leetcode3/\354\235\264\354\247\204\355\235\254/315. Count of Smaller Numbers After Self.java"
new file mode 100644
index 00000000..2725a47c
--- /dev/null
+++ "b/leetcode3/\354\235\264\354\247\204\355\235\254/315. Count of Smaller Numbers After Self.java"	
@@ -0,0 +1,81 @@
+/*
+
+1. 아이디어 : 머지 소트 방식 활용. 병합정렬중 Merge 단계에서, 왼쪽 배열의 값이 오른쪽 배열의 값보다 크다면, 
+             오른쪽 배열의 값이 왼쪽 배열의 값보다 작다는 의미이므로, count를 증가시킨다
+
+2. 시간복잡도 : O(NlogN)
+
+3. 자료구조/알고리즘 : 머지소트
+
+ */
+
+class Solution {
+    private int[] count;
+
+    private class Pair {
+        int val;
+        int index;
+
+        Pair(int val, int index) {
+            this.val = val;
+            this.index = index;
+        }
+    }
+
+    public List<Integer> countSmaller(int[] nums) {
+        int n = nums.length;
+        count = new int[n];
+        
+        Pair[] pairs = new Pair[n];
+        for (int i = 0; i < n; i++) {
+            pairs[i] = new Pair(nums[i], i);
+        }
+
+        mergeSort(pairs, 0, n - 1);
+
+        List<Integer> result = new ArrayList<>();
+        for (int c : count) {
+            result.add(c);
+        }
+        return result;
+    }
+
+    private void mergeSort(Pair[] pairs, int left, int right) {
+        if (left >= right) return;
+
+        int mid = left + (right - left) / 2;
+        mergeSort(pairs, left, mid);
+        mergeSort(pairs, mid + 1, right);
+
+        merge(pairs, left, mid, right);
+    }
+
+    private void merge(Pair[] pairs, int left, int mid, int right) {
+        Pair[] temp = new Pair[right - left + 1];
+        int i = left;
+        int j = mid + 1;
+        int k = 0;
+        int rightCount = 0;
+
+        while (i <= mid && j <= right) {
+            if (pairs[j].val < pairs[i].val) {
+                rightCount++;
+                temp[k++] = pairs[j++];
+            } else {
+                count[pairs[i].index] += rightCount;
+                temp[k++] = pairs[i++];
+            }
+        }
+
+        while (i <= mid) {
+            count[pairs[i].index] += rightCount;
+            temp[k++] = pairs[i++];
+        }
+
+        while (j <= right) {
+            temp[k++] = pairs[j++];
+        }
+
+        System.arraycopy(temp, 0, pairs, left, temp.length);
+    }
+}
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