Merge pull request #1983 from hoxiansen/hoxiansen-pr

Update 0518.零钱兑换II.md 补充二维dp数组的Java代码

problems/0518.零钱兑换II.md

@@ -215,6 +215,27 @@ class Solution {
     }
 }
 ```
+```Java
+// 二维dp数组版本，方便理解
+class Solution {
+    public int change(int amount, int[] coins) {
+        int[][] dp = new int[coins.length][amount + 1];
+        // 只有一种硬币的情况
+        for (int i = 0; i <= amount; i += coins[0]) {
+            dp[0][i] = 1;
+        }
+        for (int i = 1; i < coins.length; i++) {
+            for (int j = 0; j <= amount; j++) {
+                // 第i种硬币使用0~k次，求和
+                for (int k = 0; k * coins[i] <= j; k++) {
+                    dp[i][j] += dp[i - 1][j - k * coins[i]];
+                }
+            }
+        }
+        return dp[coins.length - 1][amount];
+    }
+}
+```
 
 Python：