Create: 1299-replace-elements-with-greatest-element-on-right-side.rb

ruby/1299-replace-elements-with-greatest-element-on-right-side.rb

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+# @param {Integer[]} arr
+# @return {Integer[]}
+
+#naive/brute force approach: time = O(n^2), space = O(n)
+def replace_elements(arr)
+    ans = []
+    (0...arr.length - 1).each do |idx|
+         ans << arr.slice(idx + 1, arr.length).max
+    end
+    ans << - 1
+    ans
+end
+
+#iterate backwards while keeping track of previous max: time = O(n), space = O(1)
+def replace_elements(arr)
+    max_right = -1
+    (arr.length - 1).downto(0).each do |idx|
+        new_max = [max_right, arr[idx]].max
+        arr[idx] = max_right
+        max_right = new_max
+    end
+    arr
+end