增加C++二分模板

Author: xybh-l

91/binary-search.md

@@ -157,10 +157,24 @@ function binarySearch(nums, target) {
 
 ##### C++
 
-暂时空缺，欢迎 [PR](https://github.com/leetcode-pp/leetcode-cheat/pulls)
-
 ```cpp
+int binarySearch(vector<int>& nums, int target){
+  if(nums.size() == 0)
+    return -1;
 
+  int left = 0, right = nums.size() - 1;
+  while(left <= right){
+    int mid = left + ((right - left) >> 1);
+    if(nums[mid] == target){ return mid; }
+    // 搜索区间变为 [mid+1, right]
+    else if(nums[mid] < target) 
+	left = mid + 1;
+    // 搜索区间变为 [left, mid - 1]
+    else 
+	right = mid - 1; 
+  }
+  return -1;
+}
 ```
 
 ### 寻找最左边的满足条件的值