add cpp algorithm

add cpp algorithm

Author: sylucky

2.selectionSort.md

@@ -16,6 +16,46 @@
 
 ![动图演示](res/selectionSort.gif)
 
+```cpp
+void changeValue(int *a, int *b)
+{
+    int temp = *a;
+    *a = *b;
+    *b = temp;
+}
+/*
+    选择排序，升序排列
+    O(N^2)时间复杂度，O(1)空间复杂度
+*/
+vector<int> AlgorithmSort::selectionSort(vector<int> vec)
+{
+    int len = vec.size();
+    int min = 0;
+    //边界值判定
+    if (len <= 1)
+        return vec;
+
+    //原始写法
+    //for (int i = 0; i < len; i++)
+    //{
+    //  for (int j = i+1; j < len; j++)
+    //  {
+    //      if (vec[i] > vec[j])
+    //          changeValue(&vec[i], &vec[j]);
+    //  }
+    //} 
+
+    //优化写法
+    for (int i = 0; i < len-1; i++)
+    {
+        // 取最小元素
+        vector<int>::iterator it = min_element(vec.begin() + i + 1, vec.end());
+        if(vec[i]>*it)
+            changeValue(&vec[i], &vec[it-vec.begin()]);
+    }
+    return vec;
+}
+```
 
 ## 3. JavaScript 代码实现