add some new file

src/Solution227.java

@@ -0,0 +1,42 @@
+import java.util.Stack;
+
+/**
+ * Created by cuibowu on 2017/8/30.
+ */
+public class Solution227 {
+    public int calculate(String s) {
+        if(s.length()==0)
+            return 0;
+        Stack<Integer> stack = new Stack<>();
+        int k=0;
+        char operator = '+';
+        for(int i=0;i<s.length();i++){
+            if(Character.isDigit(s.charAt(i))){
+                k = k*10+s.charAt(i)-'0';
+            }
+            if(!Character.isDigit(s.charAt(i))&&s.charAt(i)!=' '||i==s.length()-1){
+                switch (operator){
+                    case '-':
+                        stack.push(-k);
+                        break;
+                    case '+':
+                        stack.push(k);
+                        break;
+                    case '*':
+                        stack.push(stack.pop()*k);
+                        break;
+                    case '/':
+                        stack.push(stack.pop()/k);
+                        break;
+                }
+                operator=s.charAt(i);
+                k=0;
+            }
+        }
+        int res=0;
+        while(!stack.isEmpty()){
+            res+=stack.pop();
+        }
+        return res;
+    }
+}

src/Solution230.java

@@ -0,0 +1,19 @@
+/**
+ * Created by cuibowu on 2017/8/30.
+ */
+public class Solution230 {
+    public int kthSmallest(TreeNode root, int k) {
+        int count = count(root.left);
+        if(k<=count)
+            return kthSmallest(root.left,k);
+        else if(k>count+1)
+            return kthSmallest(root.right,k-1-count);
+        return root.val;
+    }
+
+    private int count(TreeNode root) {
+        if (root == null)
+            return 0;
+        return count(root.left)+count(root.right)+1;
+    }
+}

src/Solution344.java

@@ -0,0 +1,18 @@
+import java.util.Arrays;
+
+/**
+ * Created by cuibowu on 2017/8/30.
+ */
+public class Solution344 {
+    public String reverseString(String s) {
+        char[] word = s.toCharArray();
+        for (int i = 0, j = s.length() - 1; i < j; ) {
+            char temp = word[i];
+            word[i] = word[j];
+            word[j] = temp;
+            i++;
+            j--;
+        }
+        return Arrays.toString(word);
+    }
+}

src/Solution53.java

@@ -0,0 +1,20 @@
+/**
+ * Created by cuibowu on 2017/8/30.
+ */
+public class Solution53 {
+    public int maxSubArray(int[] nums) {
+        int[] dp = new int[nums.length];
+        dp[0]=nums[0];
+        int max = nums[0];
+        for(int i=1;i<nums.length;i++){
+            if(dp[i-1]>0){
+                dp[i]=nums[i]+dp[i-1];
+            }
+            else
+                dp[i]=nums[i];
+            max=Math.max(max,dp[i]);
+        }
+        return max;
+    }
+
+}