[N-0] add 696

Author: fishercoder1534

README.md

@@ -23,6 +23,7 @@ Your ideas/fixes/algorithms are more than welcome!
 |  #  |      Title     |   Solutions   | Time          | Space         | Difficulty  | Tag          | Notes
 |-----|----------------|---------------|---------------|---------------|-------------|--------------|-----
 |697|[Degree of an Array](https://leetcode.com/problems/degree-of-an-array/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_697.java) | O(n) | O(n) | Easy | 
+|696|[Count Binary Substrings](https://leetcode.com/problems/count-binary-substrings/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_696.java) | O(n) | O(n) | Easy | 
 |695|[Max Area of Island](https://leetcode.com/problems/max-area-of-island/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_695.java) | O(m*n) | O(1) | Easy | DFS
 |694|[Number of Distinct Islands](https://leetcode.com/problems/number-of-distinct-islands/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_694.java) | O(m*n) | O(1) | Medium | DFS
 |693|[Binary Number with Alternating Bits](https://leetcode.com/problems/binary-number-with-alternating-bits/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_693.java) | O(n) | O(1) | Easy | 

src/main/java/com/fishercoder/solutions/_696.java

@@ -0,0 +1,63 @@
+package com.fishercoder.solutions;
+
+/**
+ * 696. Count Binary Substrings
+ *
+ * Give a string s, count the number of non-empty (contiguous) substrings that have the same number of 0's and 1's,
+ * and all the 0's and all the 1's in these substrings are grouped consecutively.
+ * Substrings that occur multiple times are counted the number of times they occur.
+
+ Example 1:
+
+ Input: "00110011"
+ Output: 6
+ Explanation: There are 6 substrings that have equal number of consecutive 1's and 0's: "0011", "01", "1100", "10", "0011", and "01".
+
+ Notice that some of these substrings repeat and are counted the number of times they occur.
+
+ Also, "00110011" is not a valid substring because all the 0's (and 1's) are not grouped together.
+
+ Example 2:
+
+ Input: "10101"
+ Output: 4
+ Explanation: There are 4 substrings: "10", "01", "10", "01" that have equal number of consecutive 1's and 0's.
+
+ Note:
+ s.length will be between 1 and 50,000.
+ s will only consist of "0" or "1" characters.
+ */
+public class _696 {
+    public static class Solution1 {
+        public int countBinarySubstrings(String s) {
+            int n = s.length();
+            /**a[i][0] denotes from most left up to i (inclusive), how many consecutive 0's
+             * a[i][1] denotes from most left up to i (inclusive), how many consecutive 1's*/
+            int[][] a = new int[n][2];
+            /**a[i][0] denotes from i (inclusive) to the most right, how many consecutive 0's
+             * b[i][0] denotes from i (inclusive) to the most right, how many consecutive 1's*/
+            int[][] b = new int[n][2];
+            for (int i = 0; i < n; i++) {
+                if (s.charAt(i) == '0') {
+                    a[i][0] = 1 + (i - 1 >= 0 ? a[i - 1][0] : 0);
+                } else {
+                    a[i][1] = 1 + (i - 1 >= 0 ? a[i - 1][1] : 0);
+                }
+            }
+            for (int i = n - 1; i >= 0; i--) {
+                if (s.charAt(i) == '0') {
+                    b[i][0] = 1 + (i + 1 < n ? b[i + 1][0] : 0);
+                } else {
+                    b[i][1] = 1 + (i + 1 < n ? b[i + 1][1] : 0);
+                }
+
+            }
+            long ans = 0;
+            for (int i = 0; i + 1 < n; i++) {
+                ans += Math.min(a[i][0], b[i + 1][1]);
+                ans += Math.min(a[i][1], b[i + 1][0]);
+            }
+            return (int) ans;
+        }
+    }
+}