Solve Two House Robber Problems

Author: bky373

house-robber-ii/bky373.java

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+/*
+    time: O(N)
+    time: O(1)
+ */
+class Solution {
+
+    public int rob(int[] nums) {
+        if (nums.length == 0) {
+            return 0;
+        }
+        if (nums.length == 1) {
+            return nums[0];
+        }
+
+        return Math.max(
+                dp(nums, 0, nums.length - 2),
+                dp(nums, 1, nums.length - 1)
+        );
+    }
+
+    private static int dp(int[] nums, int start, int end) {
+        int maxOfOneStepAhead = nums[end];
+        int maxOfTwoStepsAhead = 0;
+
+        for (int i = end - 1; i >= start; --i) {
+            int curr = Math.max(maxOfOneStepAhead, maxOfTwoStepsAhead + nums[i]);
+
+            maxOfTwoStepsAhead = maxOfOneStepAhead;
+            maxOfOneStepAhead = curr;
+        }
+        return maxOfOneStepAhead;
+    }
+}

house-robber/bky373.java

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+/*
+    time: O(N)
+    space: O(1)
+ */
+class Solution {
+
+    public int rob(int[] nums) {
+        if (nums.length == 0) {
+            return 0;
+        }
+
+        int maxOfTwoStepsAhead = 0;
+        int maxOfOneStepAhead = nums[nums.length - 1];
+
+        for (int i = nums.length - 2; i >= 0; --i) {
+            int curr = Math.max(maxOfOneStepAhead, maxOfTwoStepsAhead + nums[i]);
+
+            maxOfTwoStepsAhead = maxOfOneStepAhead;
+            maxOfOneStepAhead = curr;
+        }
+        return maxOfOneStepAhead;
+    }
+}