add code of 34

Author: Li Li

Algorithms Basics/Chapter 6. Binary Search/34. Search for a Range.cs

@@ -0,0 +1,42 @@
+// direct thought
+public class Solution {
+    public int[] SearchRange(int[] nums, int target) {
+        int[] res = new int[2] { -1, -1 };
+        if (nums == null || nums.Length == 0) return res;
+        int start = FindLeftIndex(nums, target);
+        // if starting point not found, then return not found res[]
+        if (start == -1) return res;
+        int end = FindRightIndex(nums, target);  // no need to check not found again.
+        res[0] = start;
+        res[1] = end;
+        return res;
+    }
+    private int FindLeftIndex(int[] nums, int target) {
+        int left = 0, right = nums.Length - 1;
+        while (left + 1 < right) {
+            int mid = left + (right - left) / 2;
+            if (nums[mid] < target) {
+                left = mid;
+            } else {
+                right = mid;
+            }
+        }
+        if (nums[left] == target) return left;
+        if (nums[right] == target) return right;
+        return -1;
+    }
+    private int FindRightIndex(int[] nums, int target) {
+        int left = 0, right = nums.Length - 1;
+        while (left + 1 < right) {
+            int mid = left + (right - left) / 2;
+            if (nums[mid] <= target) {
+                left = mid;
+            } else {
+                right = mid;
+            }
+        }
+        if (nums[right] == target) return right;
+        if (nums[left] == target) return left;
+        return -1;
+    }
+}