Create 1639-number-of-ways-to-form-a-target-string-given-a-dictionary.js

Author: everthis

1639-number-of-ways-to-form-a-target-string-given-a-dictionary.js

@@ -0,0 +1,34 @@
+/**
+ * @param {string[]} words
+ * @param {string} target
+ * @return {number}
+ */
+const numWays = function (words, target) {
+  const m = words[0].length
+  const n = target.length
+  const memo = Array.from({ length: m }, () => Array(n))
+  const charAtIndexCnt = Array.from({ length: 128 }, () => Array(m).fill(0))
+  const mod = 10 ** 9 + 7
+  for (let word of words) {
+    for (let i = 0; i < m; i++) {
+      charAtIndexCnt[word.charCodeAt(i)][i] += 1
+    }
+  }
+
+  return dp(0, 0)
+  function dp(k, i) {
+    // found one
+    if (i == n) return 1
+    // not found
+    if (k == m) return 0
+    if (memo[k][i] != null) return memo[k][i]
+    const c = target.charCodeAt(i)
+    // skip k_th char
+    let ans = dp(k + 1, i)
+    if (charAtIndexCnt[c][k] > 0) {
+      ans += dp(k + 1, i + 1) * charAtIndexCnt[c][k]
+      ans %= mod
+    }
+    return (memo[k][i] = ans)
+  }
+}