Create 730-count-different-palindromic-subsequences.js

Author: everthis

730-count-different-palindromic-subsequences.js

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+/**
+ * @param {string} S
+ * @return {number}
+ */
+const countPalindromicSubsequences = function(S) {
+  const len = S.length
+  const dp = Array.from({ length: len }, () => new Array(len).fill(0))
+  const mod = 10 ** 9 + 7
+  for (let i = 0; i < len; i++) dp[i][i] = 1
+  for (let distance = 1; distance < len; distance++) {
+    for (let i = 0; i < len - distance; i++) {
+      let j = i + distance
+      if (S[i] === S[j]) {
+        let low = i + 1
+        let high = j - 1
+        while (low <= high && S[low] != S[j]) low++
+        while (low <= high && S[high] != S[j]) high--
+        if (low > high) {
+          dp[i][j] = dp[i + 1][j - 1] * 2 + 2
+        } else if (low == high) {
+          dp[i][j] = dp[i + 1][j - 1] * 2 + 1
+        } else {
+          dp[i][j] = dp[i + 1][j - 1] * 2 - dp[low + 1][high - 1]
+        }
+      } else {
+        dp[i][j] = dp[i][j - 1] + dp[i + 1][j] - dp[i + 1][j - 1]
+      }
+      dp[i][j] = dp[i][j] < 0 ? dp[i][j] + mod : dp[i][j] % mod
+    }
+  }
+  return dp[0][len - 1]
+}