### man.god96's blog

By man.god96, history, 2 months ago, ,

This is regarding https://leetcode.com/problems/coin-change/. It is a simple DP problem. And I wrote the following code for it using map for memoization. But it gave Time limit extended -

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


But when I changed map to vector, it gets ACCEPTED (Following code got accepted)-

class Solution {
public:
int coinChange(vector<int>& coins, int amount) {
int Max = amount + 1;
vector<int> dp(amount + 1, Max);
dp[0] = 0;
for (int i = 1; i <= amount; i++) {
for (int j = 0; j < coins.size(); j++) {
if (coins[j] <= i) {
dp[i] = min(dp[i], dp[i - coins[j]] + 1);
}
}
}
return dp[amount] > amount ? -1 : dp[amount];
}
};


So why map gave TLE and not vector. When should we use map and when vector while solving DP problems? Any help will be appreciated.

• -14

 » 2 months ago, # |   +2 In a map, all operations (accessing an element, creating a new element, etc.) take $O(\log n)$ time to execute (where $n$ is the size of the container). In a vector, the same operations take $O(1)$ time.
 » 2 months ago, # |   0 O(logN) != O(1)
 » 2 months ago, # | ← Rev. 2 →   0 for further reading, refer to Antti Laaksonen, Guide to Competitive Programming , Chapter 5, Section 3, Experiments there are comparison tables, and explained in decent way
 » 2 months ago, # |   0 Yeah right guys. Thanks. "The C++ standard library contains two map implementations that correspond to the set implementations: the structure map is based on a balanced binary tree and accessing elements takes O(logn) time, while the structure unordered_map uses hashing and accessing elements takes O(1) time on average."