三个递归算法：1.用递归算法求前 n 个正整数的和；2.用递归算法求整形数组中前 n 个元素的和；3.递归实现全排列。-Three recursive algorithms: 1. Before seeking using recursive and n positive integers 2. With a recursive algorithm to solve the integer array of n elements and before 3. Recursive full ar