Design Pattern 12: Modified Binary Search | Problem: Find Peak Element

12_find_peak_element.cpp

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+/* Problem: https://leetcode.com/problems/find-peak-element/
+ */
+
+#include <iostream>
+#include <vector>
+
+int solution(const std::vector<int> &nums) {
+  if (nums.size() == 0) {
+    return -1;
+  }
+  if (nums.size() == 1) {
+    return 0;
+  };
+  long long first = 0, last = nums.size() - 1, pi = last / 2;
+  while (true) {
+    if (pi == 0) {
+      if (nums[pi] > nums[pi + 1]) {
+        return pi;
+      } else {
+        first = pi + 1;
+        pi = ((last - first) / 2) + first;
+        continue;
+      }
+    } else if (pi == (long long)nums.size() - 1) {
+      if (nums[pi] > nums[pi - 1]) {
+        return pi;
+      } else {
+        last = pi - 1;
+        pi = ((last - first) / 2) + first;
+        continue;
+      }
+    }
+    if (nums[pi] < nums[pi - 1]) {
+      last = pi - 1;
+    } else if (nums[pi] < nums[pi + 1]) {
+      first = pi + 1;
+    } else {
+      return pi;
+    }
+    pi = ((last - first) / 2) + first;
+  }
+
+  return pi;
+}
+
+int main() {
+  std::vector<int> nums1 = {1, 2, 3, 1}, nums2 = {1, 2, 1, 3, 5, 6, 4};
+  int i1, i2;
+  i1 = solution(nums1);
+  std::cout << i1 << " : " << nums1[i1] << '\n';
+  i2 = solution(nums2);
+  std::cout << i2 << " : " << nums2[i2] << '\n';
+}