744.find-smallest-letter-greater-than-target(date 2019-06-17)

daily/2019-06-17.md

@@ -0,0 +1,84 @@
+# 毎日一题 -  744. find smallest letter greater than target
+
+## 信息卡片
+* 时间：2019-06-17
+* 题目链接：https://leetcode.com/problems/find-smallest-letter-greater-than-target/
+* tag：`Array`
+
+## 题目描述
+```
+Given a list of sorted characters letters containing only lowercase letters, and given a target letter target, find the smallest element in the list that is larger than the given target.
+
+Letters also wrap around. For example, if the target is target = 'z' and letters = ['a', 'b'], the answer is 'a'.
+
+Examples:
+    Input:
+    letters = ["c", "f", "j"]
+    target = "a"
+    Output: "c"
+
+    Input:
+    letters = ["c", "f", "j"]
+    target = "c"
+    Output: "f"
+
+    Input:
+    letters = ["c", "f", "j"]
+    target = "d"
+    Output: "f"
+
+    Input:
+    letters = ["c", "f", "j"]
+    target = "g"
+    Output: "j"
+
+    Input:
+    letters = ["c", "f", "j"]
+    target = "j"
+    Output: "c"
+
+    Input:
+    letters = ["c", "f", "j"]
+    target = "k"
+    Output: "c"
+Note:
+    letters has a length in range [2, 10000].
+    letters consists of lowercase letters, and contains at least 2 unique letters.
+    target is a lowercase letter.
+```
+
+## 思路
+二分查找，提高速度
+要求是查找某一个元素，又是在有序的集合中。
+所以我们可以用二分查找
+1. 排除两种情况；target 小于首元素|| target 大于等于尾元素 => 目标都是首元素
+2. 当target>=letters[mid] 时(我们要的值一定在右边)，调整左区间 min = mid+1;
+3. 当target< letters[mid] 时，调整右区间 max = mid-1;
+4. 循环终止条件是 min > max; 最终返回min位置元素
+
+## 建议
+在leetcode上找一个数组稍微长一点的测试用例，在纸上画出整个过程；对理解很有帮助
+
+## 参考答案
+```js
+/**
+ * @param {character[]} letters
+ * @param {character} target
+ * @return {character}
+ */
+var nextGreatestLetter = function(letters, target) {
+    const length = letters.length
+    let min = 0;
+    let max = length - 1;
+    if(target >= letters[length-1] || target < letters[0]) return letters[0];
+    while(min <= max) {
+        const mid = (max+min) >> 1
+        if(target >= letters[mid]) {
+            min = mid + 1;
+        } else {
+            max = mid - 1;
+        }
+    }
+    return letters[min]
+};
+```

daily/README.md

@@ -60,6 +60,12 @@ tag: `tree`
 
 时间: 2019-06-14
 
+### [744.find-smallest-letter-greater-than-target](./2019-06-17.md)
+
+tag：`Array` `binary search`  
+
+时间：2019-06-17
+
 ### [letter-combinations-of-a-phone-number](./2019-06-18.md)
 
 tag: `backtrack`
@@ -78,7 +84,6 @@ tag：`Array`
 
 时间：2019-06-20
 
-
 ### [nth-highest-salary](./2019-06-21.md)
 
 tag: `sql`