Merge pull request #72 from Jer3myYu/java-solutions-greedy

Java Chapter 16: Greedy

Author: Destiny-02

java/Greedy/Candies.java

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+import java.util.Arrays;
+
+public class Candies {
+    public int candies(int[] ratings) {
+        int n = ratings.length;
+        // Ensure each child starts with 1 candy.
+        int[] candies = new int[n];
+        Arrays.fill(candies, 1);
+        // First pass: for each child, ensure the child has more candies  
+        // than their left-side neighbor if the current child's rating is 
+        // higher.
+        for (int i = 1; i < n; i++) {
+            if (ratings[i] > ratings[i - 1]) {
+                candies[i] = candies[i - 1] + 1;
+            }
+        }
+        // Second pass: for each child, ensure the child has more candies 
+        // than their right-side neighbor if the current child's rating is 
+        // higher.
+        for (int i = n - 2; i >= 0; i--) {
+            if (ratings[i] > ratings[i + 1]) {
+                // If the current child already has more candies than their 
+                // right-side neighbor, keep the higher amount.
+                candies[i] = Math.max(candies[i], candies[i + 1] + 1);
+            }
+        }
+        return Arrays.stream(candies).sum();
+    }
+}

java/Greedy/GasStations.java

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+import java.util.Arrays;
+
+public class GasStations {
+    public int gasStations(int[] gas, int[] cost) {
+        // If the total gas is less than the total cost, completing the
+        // circuit is impossible.
+        if (Arrays.stream(gas).sum() < Arrays.stream(cost).sum()) {
+            return -1;
+        }
+        int start, tank;
+        start = tank = 0;
+        for (int i = 0; i < gas.length; i++) {
+            tank += gas[i] - cost[i];
+            // If our tank has negative gas, we cannot continue through the 
+            // circuit from the current start point, nor from any station 
+            // before or including the current station 'i'.
+            if (tank < 0) {
+                // Set the next station as the new start point and reset the 
+                // tank.
+                start = i + 1;
+                tank = 0;
+            }
+        }
+        return start;
+    }
+}

java/Greedy/JumpToTheEnd.java

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+public class JumpToTheEnd {
+    public boolean jumpToTheEnd(int[] nums) {
+        // Set the initial destination to the last index in the array.
+        int destination = nums.length - 1;
+        // Traverse the array in reverse to see if the destination can be 
+        // reached by earlier indexes.
+        for (int i = nums.length - 1; i >= 0; i--) {
+            // If we can reach the destination from the current index,
+            // set this index as the new destination.
+            if (i + nums[i] >= destination) {
+                destination = i;
+            }
+        }
+        // If the destination is index 0, we can jump to the end from index 
+        // 0.
+        return destination == 0;
+    }
+}