work

70works/week6/work/decode_ways.js

+/**
+ * @param {string} s
+ * @return {number}
+ */
+var numDecodings = function(s) {
+    if(!s) return 0;
+    let len = s.length;
+    let dp = new Array(len + 1).fill(0);
+    dp[0]=1;
+    dp[1] = s[0] === '0'? 0 : 1;
+    for(let i = 2; i <= len; i++) {
+        if(s[i - 1] !== '0') {
+            dp[i] += dp[i-1];
+        }
+        if(s[i - 2] === '1' ||(s[i - 2] === '2' && s[i - 1] >= 0 && s[i - 1] <= 6)) {
+            dp[i] += dp[i-2];
+        }
+    }
+    return dp[len];
+    
+    };
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