1AC. Intuitive DP. But please note the if condition, there's a trick - we cannot build upon an invalid dp slot.

class Solution {public:    int coinChange(vector
     
      & coins, int amount) {        int n = coins.size();        if (!n) return -1;        vector
      
        dp(amount + 1, INT_MAX);        dp[0] = 0;                for (int i = 1; i <= amount; i ++)        for (auto c: coins)        {            int prev = i - c;            if (i >= c && (prev?dp[prev]!=INT_MAX : true))            {                dp[i] = min(dp[i], dp[prev] + 1);            }        }                return (dp[amount] == INT_MAX) ? -1 : dp[amount];    }};