[feat ]: leetcode 42 with rust

Author: BWbwchen

leetcode/42.cpp renamed to leetcode/0042_Trapping-Rain-Water.cpp

@@ -0,0 +1,47 @@
+impl Solution {
+    pub fn trap(height: Vec<i32>) -> i32 {
+        // suppose that the left of an unknow sequence is the level of the water.
+        let mut boundary_idx = 0;
+        let mut curr_water_level = *height.first().unwrap();
+        let mut ret = 0;
+        let mut acc_water = 0;
+
+        for (i, &h) in height.iter().enumerate() {
+            if curr_water_level <= h {
+                // we have completed a trap of water.
+                println!("Accumulate {}, from {} to {}", acc_water, boundary_idx, i);
+                boundary_idx = i;
+                curr_water_level = h;
+                ret += acc_water;
+                acc_water = 0;
+            } else {
+                // continuously accumulate water
+                acc_water += curr_water_level - h;
+            }
+        }
+
+        // But in some case, the above assumption doesn't hold, we should use the other endpoint as the level of the water.
+        // when `acc_water != 0`, it means that this trap of water didn't have a correct endpoint.
+        if acc_water != 0 {
+            acc_water = 0;
+            curr_water_level = *height.last().unwrap();
+            let old_boundary_idx = boundary_idx;
+            boundary_idx = height.len() - 1;
+            for (i, &h) in height.iter().enumerate().rev() {
+                if i < old_boundary_idx {
+                    break;
+                }
+                if curr_water_level <= h {
+                    println!("*Accumulate {}, from {} back {}", acc_water, i, boundary_idx);
+                    curr_water_level = h;
+                    ret += acc_water;
+                    acc_water = 0;
+                    boundary_idx = i;
+                } else {
+                    acc_water += curr_water_level - h;
+                }
+            }
+        }
+        ret
+    }
+}