Create 1987-number-of-unique-good-subsequences.js

Author: everthis

1987-number-of-unique-good-subsequences.js

@@ -0,0 +1,52 @@
+/**
+ * @param {string} binary
+ * @return {number}
+ */
+const numberOfUniqueGoodSubsequences = function (binary) {
+  const n = binary.length,
+    P = 1e9 + 7
+  let first1Position = -1,
+    first0Position = -1
+  for (let i = 0; i < binary.length; i++) {
+    if (binary[i] === '0' && first0Position == -1) {
+      first0Position = i
+    }
+    if (binary[i] === '1' && first1Position == -1) {
+      first1Position = i
+    }
+    if(first0Position !== -1 && first1Position !== -1) break
+  }
+  if (first1Position === -1) return 1
+  if (first0Position === -1) return n
+
+  const next0 = new Array(n).fill(0)
+  const next1 = new Array(n).fill(0)
+  let nextZero = -1,
+    nextOne = -1
+  for (let i = binary.length - 1; i >= 0; i--) {
+    next0[i] = nextZero
+    next1[i] = nextOne
+    if (binary[i] === '0') {
+      nextZero = i
+    } else {
+      nextOne = i
+    }
+  }
+  const dp = new Array(n).fill(-1)
+  return (1 + fn(first1Position)) % P
+
+  function fn(index) {
+    if (index == n) return 0
+    if (dp[index] !== -1) return dp[index]
+    let result = 1
+    if (next0[index] >= 0) {
+      result += fn(next0[index])
+      result %= P
+    }
+    if (next1[index] >= 0) {
+      result += fn(next1[index])
+      result %= P
+    }
+    return (dp[index] = result)
+  }
+}