add solution of problem 120: triangle

Author: ppulse

Triangle120/README.md

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+Given a triangle, find the minimum path sum from top to bottom. Each step you may move to adjacent numbers on the row below.
+
+For example, given the following triangle
+```
+[
+     [2],
+    [3,4],
+   [6,5,7],
+  [4,1,8,3]
+]
+```
+The minimum path sum from top to bottom is `11` (i.e., 2 + 3 + 5 + 1 = 11).
+
+####Note:
+Bonus point if you are able to do this using only *O(n)* extra space, where *n* is the total number of rows in the triangle.

Triangle120/Solution.java

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+public class Solution {
+    public int minimumTotal(List<List<Integer>> triangle) {
+        if (triangle == null || triangle.size() == 0)
+            return 0;
+        
+        int size = triangle.size();
+        int[] sum = new int[size];
+        
+        List<Integer> lastList = triangle.get(size - 1);
+        
+        int idx = 0;
+        for (int val : lastList) {
+            sum[idx++] = val;
+        }
+        
+        for (int i = size - 2; i >= 0; i--) {
+            List<Integer> ls = triangle.get(i);
+            int lsSize = ls.size();
+            for (int j = 0; j < lsSize; j++) {
+                int minValue = sum[j] < sum[j+1] ? sum[j] : sum[j+1];
+                sum[j] = ls.get(j) + minValue; 
+            }
+        }
+        
+        return sum[0];
+    }
+}