添加（0647.回文子串.md）：增加typescript版本

Author: xiaofei-2020

problems/0647.回文子串.md

@@ -406,6 +406,63 @@ const countSubstrings = (s) => {
 }
 ```
 
+TypeScript：
+
+> 动态规划：
+
+```typescript
+function countSubstrings(s: string): number {
+    /**
+        dp[i][j]: [i,j]区间内的字符串是否为回文(左闭右闭)
+     */
+    const length: number = s.length;
+    const dp: boolean[][] = new Array(length).fill(0)
+        .map(_ => new Array(length).fill(false));
+    let resCount: number = 0;
+    // 自下而上，自左向右遍历
+    for (let i = length - 1; i >= 0; i--) {
+        for (let j = i; j < length; j++) {
+            if (
+                s[i] === s[j] &&
+                (j - i <= 1 || dp[i + 1][j - 1] === true)
+            ) {
+                dp[i][j] = true;
+                resCount++;
+            }
+        }
+    }
+    return resCount;
+};
+```
+
+> 双指针法：
+
+```typescript
+function countSubstrings(s: string): number {
+    const length: number = s.length;
+    let resCount: number = 0;
+    for (let i = 0; i < length; i++) {
+        resCount += expandRange(s, i, i);
+        resCount += expandRange(s, i, i + 1);
+    }
+    return resCount;
+};
+function expandRange(s: string, left: number, right: number): number {
+    let palindromeNum: number = 0;
+    while (
+        left >= 0 && right < s.length &&
+        s[left] === s[right]
+    ) {
+        palindromeNum++;
+        left--;
+        right++;
+    }
+    return palindromeNum;
+}
+```
+
+
+
 
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