1. 先找一個(gè)基準(zhǔn)數(shù)base，通常選擇最左邊（）或者最右邊（）

2. 找到這個(gè)基準(zhǔn)數(shù)在數(shù)組中的位置(從小到大排序)

a. 兩個(gè)flag拟糕，一個(gè)從最左端開始（flag_low = left）,一個(gè)從最右端開始(flag_high = right)七咧，一個(gè)向右和措，一個(gè)向左钉嘹，直到兩個(gè)flag相遇
b. 如果nums[low] <= base 剂娄，則代表nums[low]的位置合理--> low++蠢涝，繼續(xù)向右尋找;
c. 如果nums[high] >= base，則代表nums[high]的位置合理--> high--，繼續(xù)向左尋找;
：如果基準(zhǔn)數(shù)是nums[left]， 則從high-- 開始趾牧，如果基準(zhǔn)數(shù)是nums[right]，則從left++開始
d. 當(dāng)兩個(gè)flag都到達(dá)不符合的位置儿咱，則交換low high指向的值
e. 當(dāng)兩個(gè)flag相遇時(shí)，就到了基準(zhǔn)數(shù)應(yīng)該在的位置场晶，nums[flag_high] = base;

3. 調(diào)用遞歸混埠，分別對(duì)基準(zhǔn)數(shù)左邊[ left, flag-1 ]和基準(zhǔn)數(shù)右邊[ flag+1, right ]的數(shù)組進(jìn)行排序

以數(shù)組最左邊為基準(zhǔn)數(shù)：nums[left]

 void quickSort (int left, int right, vector<int>& nums) {
     if (left >= right)  return;
     // find base num
     int base = nums[left];
     // two flags. flag_low & flag_high
     int flag_low = left;
     int flag_high = right;
     while (flag_low < flag_high) {
         while (nums[flag_high] >= base && flag_low < flag_high) {
             flag_high--;
         }
         while (nums[flag_low] <= base && flag_low < flag_high) {
             flag_low++;
         }
         if (flag_low < flag_high) {
             int tmp = nums[flag_low];
             nums[flag_low] =  nums[flag_high];
             nums[flag_high] = tmp;
        }
     }
     nums[left] = nums[flag_low];
     int flag = flag_high;
     nums[flag] = base;  //flag_high = flag_low;

     //遞歸
     quickSort(left, flag-1, nums);
     quickSort(flag+1, right, nums);
}

以數(shù)組最右邊為基準(zhǔn)數(shù)：nums[right]

 void quickSort (int left, int right, vector<int>& nums) {
     if (left >= right)  return;
     // find base num
     int base = nums[right];
     // two flags. flag_low & flag_high
     int flag_low = left;
     int flag_high = right;
     while (flag_low < flag_high) {
         while (nums[flag_low] <= base && flag_low < flag_high) {
             flag_low++;
         }
         while (nums[flag_high] >= base && flag_low < flag_high) {
             flag_high--;
         }
         if (flag_low < flag_high) {
             int tmp = nums[flag_low];
             nums[flag_low] =  nums[flag_high];
             nums[flag_high] = tmp;
        }
     }
     int flag = flag_high;
     nums[right] = nums[flag];
     nums[flag] = base;  //flag_high = flag_low;

     //遞歸
     quickSort(left, flag-1, nums);
     quickSort(flag+1, right, nums);
}

以左邊為基準(zhǔn)，返回從大到小排序結(jié)果

void quickSort (int left, int right, vector<int>& nums) {
     if (left >= right)  return;
     // find base num
     int base = nums[left];
     // two flags. flag_low & flag_high
     int flag_low = left;
     int flag_high = right;
     while (flag_low < flag_high) {
         while (nums[flag_high] <= base && flag_low < flag_high) {
             flag_high--;
         }
         while (nums[flag_low] >= base && flag_low < flag_high) {
             flag_low++;
         }
         if (flag_low < flag_high) {
             int tmp = nums[flag_low];
             nums[flag_low] =  nums[flag_high];
             nums[flag_high] = tmp;
        }
     }
     int flag = flag_high;
     nums[left] = nums[flag];
     nums[flag] = base;  //flag_high = flag_low;

     //遞歸
     quickSort(left, flag-1, nums);
     quickSort(flag+1, right, nums);
}

1. 排序算法是穩(wěn)定的嗎诗轻？
   不穩(wěn)定
   排序算法的穩(wěn)定性是說钳宪，如果在數(shù)組中，存在a[i] = a[j]扳炬，那在排序的過程中吏颖，如果a[i]和a[j]的前后順序不會(huì)發(fā)生改變，則代表它是穩(wěn)定的恨樟；如果順序會(huì)發(fā)生改變半醉，則不穩(wěn)定。
2. 排序算法的時(shí)間復(fù)雜度
   最壞：O（N^2）劝术，平均復(fù)雜度是O（N*log(N)）
   完成一次遍歷的復(fù)雜度是N缩多，需要遍歷的次數(shù)呆奕，最多是N，最少是log(N)