Given a run-length encoded string in the format "char+count" pairs, decode it to the original string by repeating each character the specified number of times. The encoded string contains alternating characters and their counts (e.g., "a3b2c1" means 3 a's, 2 b's, and 1 c).

Return null for null input and empty string for empty input.

Examples:

Input: encoded = "a3b2c1" Output: "aaabbc" Explanation: Decode to 3 a's, 2 b's, and 1 c Input: encoded = "x5" Output: "xxxxx" Explanation: Decode to 5 x's Input: encoded = "a1b1c1" Output: "abc" Explanation: Each character appears once Input: encoded = null Output: null Explanation: Return null for null input Input: encoded = "" Output: "" Explanation: Return empty string for empty input
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