add js solution for countSubstrings

problems/0647.回文子串.md

@@ -361,7 +361,51 @@ func countSubstrings(s string) int {
 }
 ```
 
+Javascript
+> 动态规划
+```javascript
+const countSubstrings = (s) => {
+    const strLen = s.length;
+    let numOfPalindromicStr = 0;
+    let dp = Array.from(Array(strLen), () => Array(strLen).fill(false));
 
+    for(let j = 0; j < strLen; j++) {
+        for(let i = 0; i <= j; i++) {
+            if(s[i] === s[j]) {
+                if((j - i) < 2) {
+                    dp[i][j] = true;
+                } else {
+                    dp[i][j] = dp[i+1][j-1];
+                }
+                numOfPalindromicStr += dp[i][j] ? 1 : 0;
+            }
+        }
+    }
+
+    return numOfPalindromicStr;
+}
+```
+
+> 双指针法：
+```javascript
+const countSubstrings = (s) => {
+    const strLen = s.length;
+    let numOfPalindromicStr = 0;
+
+    for(let i = 0; i < 2 * strLen - 1; i++) {
+        let left = Math.floor(i/2);
+        let right = left + i % 2;
+
+        while(left >= 0 && right < strLen && s[left] === s[right]){
+            numOfPalindromicStr++;
+            left--;
+            right++;
+        }
+    }
+
+    return numOfPalindromicStr;
+}
+```
 
 
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