# Greedy Algorithm

• Greedy algorithms are applied to optimization algorithms only.

• It picks always the current best solution; no backtracking.

• Not necessarily optimal --> need to prove it.

### Optimization Problem

• find a set of items that can

• meet some constraints, and

• optimize some objective function

• examples: shortest path, minimum spanning tree

• not an optimization problem: sorting

### Prove the Optimality

To prove the optimality of a greedy strategy, we need to prove the following two parts.

• Greedy Choice

• find the choice that is part of SOME optimal solution

• there may exist multiple optimal solutions; no need to prove it for ANY optimal solution

• Optimal Substructure

• After making the first choice, the final best solution is first choice + best solution for the rest of (compatible) input

• We can solve the same optimization problem recursively!

• Otherwise, we find a contradiction!

### Example problems

• activity selection (pick the earliest-finish task)

• minimum spanning tree

• Huffman code

• Dijkastra's shortest path

• graph coloring

• traveling salesman

### Greedy vs. Dynamic Programming

• Greedy: always pick the current best

• DP: pick the best based on history

• You can always use DP to solve Activity Selection problem, but it is much costly (since you have to try all other branches)

### Example Code: Easy Swaps to Sort 123

``````#include <iostream>
#include <vector>
#include <algorithm>

int main(){
int n;
std::cin >> n;
std::vector<int> candies(n);
for (int k = 0; k < n; ++k)
std::cin >> candies[k];

// count total number of 1s, 2s, 3s
int sum1 = 0, sum2 = 0, sum3 = 0;
for (int k = 0; k < n; ++k){
if (candies[k] == 1)
sum1++;
else if (candies[k] == 2)
sum2++;
else
sum3++;
}

// all 3s should be placed in the range [sum1+sum2, n-1]
// and then leave a subproblem of 1s and 2s in the range [0, sum1+sum2-1]
int swap = 0;
std::vector<int> temp_queue;
for (int k = sum1 + sum2; k < n; ++k){
if (candies[k] != 3){
swap++;
temp_queue.push_back(candies[k]);
}
}

// move all 1s to the front and 2s to the end
std::sort(temp_queue.begin(), temp_queue.end());

// in [0, sum1+sum2-1], we want to put 1s to the front and 2s to the end
for (int k = sum1 + sum2 - 1; k >= 0; --k){
if (candies[k] == 3){
candies[k] = temp_queue.back();
temp_queue.pop_back();
}
}

// finally, count how many 2s are still there in the range [0, sum1-1]
// which is supposed to be all 1s
for (int k = 0; k < sum1; ++k){
if (candies[k] == 2)
swap++;
}

std::cout << swap << std::endl;

return 0;
}``````

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