Add the dart implementation of 0704.

Author: yefeidd

problems/0704.二分查找.md

@@ -755,8 +755,56 @@ object Solution {
   }
 }
 ```
+**Dart:**
 
 
+
+```dart
+（版本一）左闭右闭区间
+class Solution {
+  int search(List<int> nums, int target) {
+    int left = 0;
+    int right = nums.length - 1;
+    while (left <= right) {
+      int middle = ((left + right)/2).truncate();
+      switch (nums[middle].compareTo(target)) {
+        case 1:
+          right = middle - 1;
+          continue;
+        case -1:
+          left = middle + 1;
+          continue;
+        default:
+          return middle;
+      }
+    }
+    return -1;
+  }
+}
+
+（版本二）左闭右开区间
+class Solution {
+  int search(List<int> nums, int target) {
+    int left = 0;
+    int right = nums.length;
+    while (left < right) {
+      int middle = left + ((right - left) >> 1);
+      switch (nums[middle].compareTo(target)) {
+        case 1:
+          right = middle;
+          continue;
+        case -1:
+          left = middle + 1;
+          continue;
+        default:
+          return middle;
+      }
+    }
+    return -1;
+  }
+}
+```
+
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