java
public class Solution {
    public String multiply(String num1, String num2) {
         if (num1.equals("0") || num2.equals("0")) {
             return "0";
           }

         int[] result = new int[num1.length() + num2.length()];

         for (int i = num1.length() - 1; i >= 0; i--) {
              for (int j = num2.length() - 1; j >= 0; j--) {
                   int mul = (num1.charAt(i) - '0') * (num2.charAt(j) - '0');
                   int p1 = i + j, p2 = i + j + 1;
                   int sum = mul + result[p2];

                   result[p2] = sum % 10;
                   result[p1] += sum / 10;
               }
           }

         StringBuilder sb = new StringBuilder();
          for (int i = 0; i < result.length; i++) {
               if (i == 0 && result[i] == 0) {
                   continue;
               }
               sb.append(result[i]);
           }

          return sb.toString();
       }
}

java
public class Solution {
    public String[] findRestaurant(String[] list1, String[] list2) {
         Map<String, Integer> indexMap = new HashMap<>();
         for (int i = 0; i < list1.length; i++) {
              indexMap.put(list1[i], i);
          }

         int minIndexSum = Integer.MAX_VALUE;
         List<String> commonRestaurants = new ArrayList<>();

         for (int i = 0; i < list2.length; i++) {
              if (indexMap.containsKey(list2[i])) { // 共同爱好
                  int indexSum = i + indexMap.get(list2[i]); // 计算索引和
                  if (indexSum < minIndexSum) { //是否是最小索引和
                      minIndexSum = indexSum;
                       commonRestaurants.clear();
                       commonRestaurants.add(list2[i]);
                   } else if (indexSum == minIndexSum) {
                       commonRestaurants.add(list2[i]);
                   }
               }
           }
        return commonRestaurants.toArray(new String[0]);
         //return commonRestaurants.toArray(new String[0]);
       }
}

java
public class Solution {
    public int coinChange(int[] coins, int amount) {
          // 初始化一个比amount大的数，表示无法凑成的金额
          int max = amount + 1;
          // 初始化dp数组，所有值设为最大值
          int[] dp = new int[amount + 1];
          Arrays.fill(dp, max);
          // 金额为0时，不需要任何硬币
          dp[0] = 0;

          // 遍历所有金额
          for (int i = 1; i <= amount; i++) {
               // 遍历所有硬币
               for (int coin : coins) {
                    // 如果当前金额大于等于硬币面值
                    if (i >= coin) {
                         // 更新dp[i]为dp[i]和dp[i - coin] + 1的最小值
                         dp[i] = Math.min(dp[i], dp[i - coin] + 1);
                    }
               }
          }

          // 如果dp[amount]没有被更新过，说明无法凑成该金额
          return dp[amount] == max ? -1 : dp[amount];
      }
}