Selection Sort: This algorithm sorts (poorly) in O(n²) where n is the size of the input array/vector.
The algorithm is fully explained, with more references, here.
The following code is a snippet of what is found at the link, but it's the most relevent piece:

Given:
Array v of n values to be sorted

Method:

for(i = 0; i < n - 1; i++) {
  min_index = i;
  for(j = i + 1; j < n; j++){
    if(v[j] < min_index){
      min_index = j;
    }
  }
  tmp = v[i];
  v[i] = v[min_index];
  v[min_index] = tmp;
}
          

选择排序定义：选择排序首先从第一次从待排序的数据元素中选出最小（或最大）的一个元素，存放在序列的起始位置，然后再从剩余的未排序元素中寻找到最小（大）元素，然后放到已排序的序列的末尾。以此类推，直到全部待排序的数据元素的个数为零。要注意的是，该算法是不稳定的排序方法。

Method:

for(i = 0; i < n - 1; i++) {
  min_index = i;
  for(j = i + 1; j < n; j++){
    if(v[j] < min_index){
      min_index = j;
    }
  }
  tmp = v[i];
  v[i] = v[min_index];
  v[min_index] = tmp;
}