feat:add four easy task

Author: Ernestchang

src/com/ernest/easy/_001.java

@@ -0,0 +1,39 @@
+package com.ernest.easy;
+
+import java.util.Arrays;
+import java.util.HashMap;
+
+public class _001 {
+
+
+    public int[] twoSum_0(int[] source, int target) {
+        for (int i = 0; i < source.length; i++) {
+            for (int j = i + 1; j < source.length; j++) {
+                if (source[i] + source[j] == target) {
+                    return new int[]{i, j};
+                }
+            }
+        }
+        return null;
+    }
+
+    public int[] twoSum_1(int[] source, int target) {
+        int length = source.length;
+        HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();
+        for (int i = 0; i < length; i++) {
+            if (map.containsKey(source[i])) {
+                return new int[]{map.get(source[i]), i};
+            }
+            map.put(target - source[i], i);
+        }
+        return null;
+    }
+
+    public static void main(String[] args) {
+        int[] source = new int[]{2, 7, 11, 15};
+        int target = 9;
+        _001 solution = new _001();
+        System.out.println(Arrays.toString(solution.twoSum_0(source, target)));
+        System.out.println(Arrays.toString(solution.twoSum_1(source, target)));
+    }
+}

src/com/ernest/easy/_007.java

@@ -0,0 +1,23 @@
+package com.ernest.easy;
+
+public class _007 {
+
+
+    public int reverse(int source) {
+        int result = 0;
+        for (; source != 0; source /= 10) {
+            result = result * 10 + source % 10;
+        }
+        return result > Integer.MAX_VALUE | result < Integer.MIN_VALUE ? 0 : result;
+    }
+
+    public static void main(String[] args) {
+        int source1 = 123;
+        int source2 = -123;
+        _007 solution = new _007();
+        System.out.println(solution.reverse(source1));
+        System.out.println(solution.reverse(source2));
+    }
+
+
+}

src/com/ernest/easy/_009.java

@@ -0,0 +1,53 @@
+package com.ernest.easy;
+
+
+public class _009 {
+
+
+    public boolean isPalindrome_0(int source) {
+        int copySource = source;
+        int reverse = 0;
+        while (source > 0) {
+            reverse = reverse * 10 + source % 10;
+            source /= 10;
+        }
+        return copySource == reverse;
+    }
+
+    /**
+     *  WAY:首先确定一条原则可行的路，全面考虑可能得情况；具体执行时，重点关注临界点，前后判断差异。
+     * @param source
+     * @return
+     */
+    public boolean isPalindrome_1(int source) {
+        if (source < 0 || (source != 0 && source % 10 == 0)) {
+            return false;
+        }
+
+        int halfReverse = 0;
+
+        /**
+         * when source is odd, source will smaller than halfReverse one bit.
+         * when source is even, source will equal with halfReverse.
+         */
+        while (source > halfReverse) {
+            // add one bit form source to halfReverse
+            halfReverse = halfReverse * 10 + source % 10;
+            // source delete the bit who is added.Add if source is odd,the middle bit will drop now.
+            source /= 10;
+        }
+
+        System.out.println("halfReverse" + halfReverse);
+        System.out.println(source);
+        return source == halfReverse || source == halfReverse / 10;
+
+    }
+
+    public static void main(String[] args) {
+
+        _009 solution = new _009();
+        System.out.println(solution.isPalindrome_0(1234321));
+        System.out.println(solution.isPalindrome_1(1234321));
+    }
+}
+

src/com/ernest/easy/_013.java

@@ -0,0 +1,38 @@
+package com.ernest.easy;
+
+import java.util.HashMap;
+import java.util.Map;
+
+public class _013 {
+
+    public int romanToInt(String s) {
+        Map<Character, Integer> map = new HashMap<>();
+        map.put('I', 1);
+        map.put('V', 5);
+        map.put('X', 10);
+        map.put('L', 50);
+        map.put('C', 100);
+        map.put('D', 500);
+        map.put('M', 1000);
+        int len = s.length();
+
+        // s.chatAt(len-1) 是最后一位，不是第一位，字符串默认，从左至右，指针依次递增
+        int sum = map.get(s.charAt(len - 1));
+
+        for (int i = len - 2; i >= 0; --i) {
+            if (map.get(s.charAt(i)) < map.get(s.charAt(i + 1))) {
+                sum -= map.get(s.charAt(i));
+            } else {
+                sum += map.get(s.charAt(i));
+            }
+        }
+        return sum;
+    }
+
+    public static void main(String[] args) {
+        _013 solution = new _013();
+        System.out.println(solution.romanToInt("IV"));
+        System.out.println(solution.romanToInt("VI"));
+        System.out.println(solution.romanToInt("IVVI"));
+    }
+}