Add two C++ solutions for 91algo day2 (leetcode821).

Author: yanglr

91algo/daily/2021-09-11_leetcode821-shortest-distance-to-a-character_naive_solution.cpp

@@ -0,0 +1,41 @@
+#include <vector>
+#include <unordered_map>
+#include <algorithm>
+#include <iostream>
+using namespace std;
+
+class Solution {
+public:
+    vector<int> shortestToChar(string s, char c) {
+        // 题意: 计算 abs[i] = indexGap(i, nearest_c), 1 <= s.length <= 10^4
+        // 输出 abs[i]的数组
+        vector<int> indexes;  // indexes of char c
+        for (int i = 0; i < s.size(); i++)
+        {
+            if (s[i] == c) indexes.push_back(i);
+        }
+        vector<int> res;
+        for (int i = 0; i < s.size(); i++)
+        {
+            vector<int> gaps;  // gap for char s[i] to char c
+            for (auto index : indexes)
+                gaps.push_back(abs(index - i));
+            auto it = min_element(gaps.begin(), gaps.end());
+            res.push_back(*it);
+        }        
+        return res;
+    }
+};
+
+// Test
+int main()
+{
+    Solution sol;
+    string s = "loveleetcode";
+    char c = 'e';
+    auto res = sol.shortestToChar(s, c);
+    for (auto &num : res)
+        cout << num << endl;
+
+    return 0;
+}

91algo/daily/2021-09-11_leetcode821-shortest-distance-to-a-character_two_pointers1.cpp

@@ -0,0 +1,55 @@
+#include <vector>
+#include <unordered_map>
+#include <algorithm>
+#include <iostream>
+using namespace std;
+
+class Solution {
+public:
+    vector<int> shortestToChar(string s, char c) {
+        vector<int> gaps(s.size());
+        vector<char> chars(s.begin(), s.end());
+
+        for (int i = 0; i < chars.size(); i++)
+        {
+            // 如果当前字符就是要搜索的字符c, 距离为 0
+            if (chars[i] == c) gaps[i] = 0;
+            else /* 否则分别向左、向右找最近的字符c */
+            {
+                int leftDistance = INT_MAX, rightDistance = INT_MAX;
+                for (int left = i; left >= 0; left--)
+                {
+                    if (chars[left] == c) // 向左找, 找到第一个
+                    {
+                        leftDistance = i - left;
+                        break;
+                    }
+                }
+                for (int right = i; right < chars.size(); right++) // 向右找, 找到第一个
+                {
+                    if (chars[right] == c)
+                    {
+                        rightDistance = right - i;
+                        break;
+                    }
+                }
+                gaps[i] = min(leftDistance, rightDistance);
+            }
+        }
+
+        return gaps;
+    }
+};
+
+// Test
+int main()
+{
+    Solution sol;
+    string s = "loveleetcode";
+    char c = 'e';
+    auto res = sol.shortestToChar(s, c);
+    for (auto &num : res)
+        cout << num << endl;
+
+    return 0;
+}