problems: add find pivot index

problems/find-pivot-index.rs

@@ -0,0 +1,20 @@
+use std::convert::TryInto;
+
+impl Solution {    
+    pub fn pivot_index(nums: Vec<i32>) -> i32 {
+        let mut from_left: i32 = 0;
+        let mut from_right: i32 = nums.iter().sum();
+        
+        for (i, e) in nums.iter().enumerate() {
+            from_right -= e;
+            
+            if from_left == from_right {
+                return i.try_into().unwrap();
+            }
+            
+            from_left += e;
+        }
+        
+        -1
+    }
+}