Solve string merge and grid path tasks

Question

Given two equal-length strings s1 and s2, create a new string by iterating i = 0 … n-1, comparing s1[i] with s2[n-1-i]: append s1[i]; if the two characters are identical append that character again, otherwise append s2[n-1-i]. Examples: s1 = "abc", s2 = "def" → "afcd"; s1 = "abp", s2 = "jda" → "aaaapj"; s1 = "abp", s2 = "abp" → "appa". Return the resulting string. Design an algorithm for a robot moving on a 2-D grid containing: walls (impassable), healing cells (+health), and monster cells (–health). Starting with a given health value, find the shortest path from start to goal such that the robot’s health never drops to 0 or negative. Output that shortest distance (or the path itself). Solve an array/string problem that requires a sliding-window technique to meet a given constraint (details unspecified in the notes).