Create 374. Guess Number Higher or Lower.md

374. Guess Number Higher or Lower.md

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+# [374. Guess Number Higher or Lower](https://leetcode.com/problems/guess-number-higher-or-lower/description/)
+# 思路
+二分法。   
+注意计算mid的时候防止溢出，见代码。
+# C++
+```
+// Forward declaration of guess API.
+// @param num, your guess
+// @return -1 if my number is lower, 1 if my number is higher, otherwise return 0
+int guess(int num);
+
+class Solution {
+public:
+    int guessNumber(int n) {
+        int low = 1, high= n, mid;
+        while(low <= high){
+            mid = low + (high - low) / 2;  // 不用 mid = (low + high) / 2, 因为这样可能溢出
+            if(guess(mid) == 0) return mid;
+            else if(guess(mid) > 0) low = mid + 1;
+            else high = mid - 1;
+        }
+    }
+};
+```