Create C8GetPostArray.md

CodingInterviewGuide/C8GetPostArray.md

@@ -0,0 +1,60 @@
+```
+package Chapter8;
+
+import java.util.HashMap;
+
+/**
+ * @author Ethan
+ * @desc 根据树的前序和中序数组生成后序数组 
+ */
+public class C8GetPostArray {
+
+	/**
+	 * @author Ethan
+	 * @desc 返回后序数组
+	 * pre[1-2-4-5-3-6-7] in[4-2-5-1-6-3-7] 
+	 */
+	public int[] getPostArray(int[] pre,int[] in){
+		//1-异常参数检测
+		if(pre == null || in == null){
+			return null;
+		}
+		//2-初始化后序数组
+		int len = pre.length;
+		int[] post = new int[len];
+		
+		//3-初始化中序数组索引
+		HashMap<Integer,Integer> map = new HashMap<Integer,Integer>();
+		for(int i = 0; i < len; i++){
+			map.put(in[i], i);
+		}
+		
+		//4-实现后序数组赋值
+		setPostIndex(pre,0,len - 1,in,0,len-1,post,len - 1,map);
+		
+		return post;
+	}
+	
+	/**
+	 * @author Ethan
+	 * @desc 设置后序数组元素 
+	 * pre[1-2-4-5-3-6-7] in[4-2-5-1-6-3-7] 
+	 */
+	public int setPostIndex(int[] pre,int pi,int pj,int[] in,int ii,int ij,
+			int[] post,int postIndex,HashMap<Integer,Integer> map){
+		//1-递归结束条件
+		if(pi > pj){
+			return postIndex;
+		}
+		//2-post数组赋值
+		post[postIndex--] = pre[pi];
+		//3-获取中序数组中头节点位置
+		int i = map.get(pre[pi]);
+		
+		//4-后序遍历先填充右子树
+		postIndex = setPostIndex(pre, pi - i + ij + 1, pj, in, i + 1, ij, post, postIndex, map);
+		
+		return setPostIndex(pre, pi + 1, pi + i - ii, in, ii, i - 1, post, postIndex, map);
+	}
+}
+```