Add Remove K Digits problem

Author: sahilbansal17

README.md

@@ -5,4 +5,5 @@ This repository contains the coding interview problems along with solutions.
 |---------------|----------|------|----------|------------|
 | [Island Count](Island%20Count) | | [CPP](Island%20Count/solution.cpp) | Graph Traversal | Medium |
 | [Minimum Falling Path Sum](Minimum%20Falling%20Path%20Sum) | [text](Minimum%20Falling%20Path%20Sum/solution.txt) | [CPP](Minimum%20Falling%20Path%20Sum/solution.cpp) | Dynamic Programming | Medium |
-| [Word Break](Word%20Break) | [text](Word%20Break/solution.txt) | [CPP](Word%20Break/solution.cpp) | Dynamic Programming | Medium |
+| [Word Break](Word%20Break) | [text](Word%20Break/solution.txt) | [CPP](Word%20Break/solution.cpp) | Dynamic Programming | Medium |
+| [Remove K Digits](Remove%20K%20Digits) | [text](Remove%20K%20Digits/solution.txt) | [CPP](Remove%20K%20Digits/solution.cpp) | Greedy, Stack | Medium |

Remove K Digits/README.md

@@ -0,0 +1,31 @@
+# Remove K Digits
+[Try the problem](https://leetcode.com/problems/remove-k-digits/)
+
+Given a non-negative integer `num` represented as a string, remove *k* digits from the number so that the new number is the smallest possible.
+
+**Note**: 
+- The given `num` does not contain any leading zero.
+
+### Example 1
+
+```
+Input: num = "1432219", k = 3
+Output: "1219"
+Explanation: Remove the three digits 4, 3, and 2 to form the new number 1219 which is the smallest.
+```
+
+### Example 2
+
+```
+Input: num = "10200", k = 1
+Output: "200"
+Explanation: Remove the leading 1 and the number is 200. Note that the output must not contain leading zeroes.
+```
+
+### Example 3
+
+```
+Input: num = "10", k = 2
+Output: "0"
+Explanation: Remove all the digits from the number and it is left with nothing which is 0.
+```

Remove K Digits/solution.cpp

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+#include <iostream>
+#include <string>
+#include <stack>
+using namespace std;
+class Solution {
+public:
+    string removeKdigits(string num, int k) {
+        int n = num.length();
+        stack<char> st;
+        for (int i = 0; i < n; ++i) {
+            if (st.empty()) {
+                st.push(num[i]);
+            } else if (st.top() > num[i] && k > 0) {
+                while (!st.empty() && st.top() > num[i] && k > 0) {
+                    st.pop();
+                    --k;
+                }
+                st.push(num[i]);
+            } else {
+                st.push(num[i]);
+            }
+        }
+        while (!st.empty() && k > 0) {
+            st.pop();
+            --k;
+        }
+        string res;
+        if (st.empty()) {
+            res = '0';
+        }
+        while (!st.empty()) {
+            res += st.top();
+            st.pop();
+        }
+        reverse(res.begin(), res.end());
+        // remove leading zeroes
+        string finalRes;
+        int firstIdx = 0;
+        int finalLen = res.length();
+        while (firstIdx < finalLen && res[firstIdx] == '0') {
+            ++firstIdx;
+        }
+        if (firstIdx == finalLen) {
+            finalRes = '0';
+        } else {
+            while (firstIdx < finalLen) {
+                finalRes += res[firstIdx];
+                ++firstIdx;
+            }
+        }
+        return finalRes;
+    }
+};
+
+int main() {
+    Solution solver;
+
+    cout << solver.removeKdigits("1234567890", 9) << endl;
+}

Remove K Digits/solution.txt

@@ -0,0 +1,2 @@
+The problem is quite tricky and revolved around the idea of comparing the numbers,
+observing some kind of properties, leading to a greedy solution.