[Flynn] week 3

climbing-stairs/flynn.cpp

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+class Solution {
+public:
+int climbStairs(int n) {
+vector<int> memo(2, 1);
+
+        for (int i = 2; i <= n; i++) {
+            memo.push_back(memo[i - 1] + memo[i - 2]);
+        }
+
+        return memo[n];
+    }
+
+};

coin-change/flynn.cpp

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+class Solution {
+public:
+int coinChange(vector<int>& coins, int amount) {
+int MAX = 10000 + 1;
+vector<int> memo(amount + 1, MAX);
+memo[0] = 0;
+
+        for (int i = 1; i <= amount; i++) {
+            for (auto coin : coins) {
+                if (i - coin >= 0) {
+                    memo[i] = min(memo[i], memo[i - coin] + 1);
+                }
+            }
+        }
+
+        return memo[amount] == MAX ? -1 : memo[amount];
+    }
+
+};

combination-sum/flynn.cpp

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+class Solution {
+public:
+vector<vector<int>> combinationSum(vector<int>& candidates, int target) {
+vector<vector<int>> res;
+queue<pair<int, pair<int, vector<int>>>> q; // {acc, {idx, combination}}
+
+        for (int i = 0; i < candidates.size(); i++) {
+            int num = candidates[i];
+
+            if (num <= target) {
+                vector<int> comb;
+                comb.push_back(num);
+                q.push({num, {i, comb}});
+            }
+
+        }
+
+        while (!q.empty()) {
+            auto p = q.front();
+            q.pop();
+
+            int acc = p.first, idx = p.second.first;
+            auto comb = p.second.second;
+
+            if (acc == target) {
+                res.push_back(comb);
+            } else if (acc < target) {
+                for (int i = idx; i < candidates.size(); i++) {
+                    int num = candidates[i];
+
+                    if (acc + num <= target) {
+                        vector<int> new_comb(comb);
+                        new_comb.push_back(num);
+                        q.push({acc + num, {i, new_comb}});
+                    }
+                }
+            }
+        }
+
+        return res;
+    }
+
+};

product-of-array-except-self/flynn.cpp

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+class Solution {
+public:
+vector<int> productExceptSelf(vector<int>& nums) {
+vector<int> res(nums.size(), 1);
+
+        for (int i = 1; i < nums.size(); i++) {
+            res[i] *= nums[i - 1] * res[i - 1];
+        }
+
+        int acc = 1;
+        for (int i = nums.size() - 2; i >= 0; i--) {
+            acc *= nums[i + 1];
+            res[i] *= acc;
+        }
+
+        return res;
+    }
+
+};

two-sum/flynn.cpp

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+class Solution {
+public:
+    vector<int> twoSum(vector<int>& nums, int target) {
+        unordered_map<int, int> past; // key: num value: index
+        vector<int> res;
+
+        past.insert({nums[0], 0});
+
+        for (int i = 1; i < nums.size(); i++) {
+            int remainder = target - nums[i];
+
+            if (past.find(remainder) != past.end()) {
+                res.push_back(i);
+                res.push_back(past[remainder]);
+                break;
+            } else {
+                past.insert({nums[i], i});
+            }
+        }
+
+        return res;
+    }
+};