Add 'Jump Game II' solution.

Author: liuyingshan

Jump_Game_II.cpp

@@ -0,0 +1,42 @@
+/*
+  Given an array of non-negative integers, you are initially positioned at the
+  first index of the array.
+  Each element in the array represents your maximum jump length at that
+  position.
+  Your goal is to reach the last index in the minimum number of jumps.
+
+  For example:
+  Given array A = [2,3,1,1,4]
+  The minimum number of jumps to reach the last index is 2.
+  (Jump 1 step from index 0 to 1, then 3 steps to the last index.)
+*/
+
+class Solution {
+public:
+    int jump(int A[], int n) {
+        if (n <= 1) return 0;
+
+        queue<int> q;  // for BFS
+        vector<int> cnt(n, 0); // c[i] represents the minimal jumps from 0 to i
+        q.push(0);
+        while (!q.empty()) {
+            int start = q.front();
+            if (start + A[start] >= n - 1) return cnt[start] + 1;
+            for (int i=A[start]; i>0; i--) {
+                if (cnt[start+i] == 0) {
+                    cnt[start+i] = cnt[start] + 1;
+                    q.push(start+i);
+                } else {
+                // if i has been reached, then i-1 must has been reached too.
+                // So just skim others.
+                // This makes sure that every position will be pushed once, be
+                // popped once and visit at most one unchanged position.
+                    break;
+                }
+            }
+            q.pop();
+        }
+
+        return -1;
+    }
+};