add: 山脉数组中查找目标值

src/array/find-in-mountain-array.js

@@ -0,0 +1,111 @@
+/**
+ * LeetCode 852. 山脉数组的峰顶索引
+ * @param {number[]} A
+ * @return {number}
+ */
+export const peakIndexInMountainArray = function (A) {
+  let left = 0
+  let right = A.length - 1
+
+  while (left < right) {
+    const mid = left + ((right - left) >> 1)
+    if (A[mid] > A[mid - 1] && A[mid] > A[mid + 1]) return mid
+    if (A[mid] < A[mid + 1] && A[mid] > A[mid - 1]) {
+      left = mid
+    } else {
+      right = mid
+    }
+  }
+
+  return 0
+}
+
+/**
+ * LintCode 585. 山脉序列中的最大值
+ * @param nums: a mountain sequence which increase firstly and then decrease
+ * @return: then mountain top
+ */
+export const mountainSequence = function (nums) {
+  let left = 0
+  let right = nums.length - 1
+
+  while (left < right) {
+    const mid = left + ((right - left) >> 1)
+    if (nums[mid] > nums[mid - 1] && nums[mid] > nums[mid + 1]) return nums[mid]
+    if (nums[mid] < nums[mid + 1] && nums[mid] > nums[mid - 1]) {
+      left = mid
+    } else {
+      right = mid
+    }
+  }
+
+  return nums[0]
+}
+
+/**
+ * // This is the MountainArray's API interface.
+ * // You should not implement it, or speculate about its implementation
+ * function MountainArray() {
+ *     @param {number} index
+ *     @return {number}
+ *     this.get = function(index) {
+ *         ...
+ *     };
+ *
+ *     @return {number}
+ *     this.length = function() {
+ *         ...
+ *     };
+ * };
+ */
+
+/**
+ * 1095. 山脉数组中查找目标值
+ * @param {number} target
+ * @param {MountainArray} mountainArr
+ * @return {number}
+ */
+export const findInMountainArray = function (target, mountainArr) {
+  let left = 0
+  let right = mountainArr.length() - 1
+
+  // 山峰索引
+  let peak = 0
+
+  while (left < right) {
+    const mid = left + ((right - left) >> 1)
+    if (mountainArr.get(mid) > mountainArr.get(mid - 1) && mountainArr.get(mid) > mountainArr.get(mid + 1)) {
+      peak = mid
+      break
+    }
+    if (mountainArr.get(mid) < mountainArr.get(mid + 1) && mountainArr.get(mid) > mountainArr.get(mid - 1)) {
+      left = mid
+    } else {
+      right = mid
+    }
+  }
+
+  // 在山峰左边查找
+  const index = binarySearch(mountainArr, target, 0, peak, v => v)
+
+  // 若存在，则直接返回下标，否则在山峰右边查找
+  return index !== -1 ? index : binarySearch(mountainArr, target, peak + 1, mountainArr.length() - 1, v => -v)
+
+  // 二分法查找
+  // 其中 fn 是用来对升序还是降序的特殊处理
+  function binarySearch (mountainArr, target, l, r, fn) {
+    target = fn(target)
+    while (l <= r) {
+      const mid = (l + (r - l) / 2) | 0
+      const cur = fn(mountainArr.get(mid))
+      if (cur === target) {
+        return mid
+      } else if (cur < target) {
+        l = mid + 1
+      } else {
+        r = mid - 1
+      }
+    }
+    return -1
+  }
+}