[PDKhan] WEEK 03 solutions

combination-sum/PDKhan.cpp

@@ -0,0 +1,27 @@
+class Solution {
+    public:
+        void backtrack(vector<int>& candidates, int target, int index, vector<int>& curr, vector<vector<int>>& result){
+            if(target < 0)
+                return;
+
+            if(target == 0){
+                result.push_back(curr);
+                return;
+            }
+
+            for(int i = index; i < candidates.size(); i++){
+                curr.push_back(candidates[i]);
+                backtrack(candidates, target-candidates[i], i, curr, result);
+                curr.pop_back();
+            }
+        }
+
+        vector<vector<int>> combinationSum(vector<int>& candidates, int target) {
+            vector<vector<int>> result;
+            vector<int> curr;
+
+            backtrack(candidates, target, 0, curr, result);
+
+            return result;
+        }
+    };

decode-ways/PDKhan.cpp

@@ -0,0 +1,28 @@
+class Solution {
+    public:
+        int numDecodings(string s) {
+            int n = s.length();
+
+            if(n == 0 || s[0] == '0')
+                return 0;
+
+            int next = 1;
+            int nextnext = 1;
+            int curr = 0;
+
+            for(int i = n - 1; i >= 0; i--){
+                if(s[i] == '0')
+                    curr = 0;
+                else{
+                    curr = next;
+                    if(i < n - 1 && (s[i] == '1' || (s[i] == '2' && s[i+1] >= '0' && s[i+1] <= '6')))
+                        curr += nextnext;
+                }
+
+                nextnext = next;
+                next = curr;
+            }
+
+            return curr;
+        }
+    };

maximum-subarray/PDKhan.cpp

@@ -0,0 +1,19 @@
+class Solution {
+    public:
+        int maxSubArray(vector<int>& nums) {
+            int cur = nums[0];
+            int max = nums[0];
+
+            for(int i = 1; i < nums.size(); i++){
+                if(nums[i] > cur + nums[i])
+                    cur = nums[i];
+                else
+                    cur = cur + nums[i];
+
+                if(max < cur)
+                    max = cur;
+            }
+
+            return max;
+        }
+    };

number-of-1-bits/PDKhan.cpp

@@ -0,0 +1,15 @@
+class Solution {
+    public:
+        int hammingWeight(int n) {
+            int cnt = 0;
+
+            while(n){
+                if(n & 1 == 1)
+                    cnt++;
+
+                n >>= 1;
+            }
+
+            return cnt;
+        }
+    };

valid-palindrome/PDKhan.cpp

@@ -0,0 +1,20 @@
+class Solution {
+    public:
+        bool isPalindrome(string s) {
+            int left = 0;
+            int right = s.length()-1;
+
+            while(left < right){
+                while(left < right && !isalnum(s[left])) left++;
+                while(left < right && !isalnum(s[right])) right--;
+
+                if(tolower(s[left]) != tolower(s[right]))
+                    return false;
+
+                left++;
+                right--;
+            }
+
+            return true;
+        }
+    };