Create 303. Range Sum Query - Immutable.md

303. Range Sum Query - Immutable.md

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+# [303. Range Sum Query - Immutable](https://leetcode.com/problems/range-sum-query-immutable/description/)
+# 思路
+用一个数组sums记录和，sums[i]代表nums[0]到nums[i]的和，那么sumRange(i, j)就应该等于sum[i] - sum[i-1], 注意单独判断i得0时。    
+注意这种面向对象的代码风格。
+# C++
+```
+class NumArray {
+private:
+    vector<int> sums;
+public:
+    NumArray(vector<int> nums) {
+        int tmp = 0;
+        for(int num: nums) {
+            tmp += num;
+            sums.push_back(tmp);
+        }
+    }
+    
+    int sumRange(int i, int j) {
+        if(i == 0) return sums[j];
+        else return sums[j] - sums[i - 1];
+    }
+};
+```